Fast multi-touch noise reduction

ABSTRACT

A low-latency touch sensitive device provides a method for determining a location of a touch event thereon. The touch sensitive device row conductors and column conductors, the path of each of the row conductors crossing the path of each of the column conductors. Each of a set of orthogonal row signals are simultaneously transmitted on a respective one of at least some of the row conductors and an amount of each of the plurality of orthogonal row signals present on each of the plurality of column conductors is detected. A set of orthogonal column signals are simultaneously transmitted on a respective one of at least some of the column conductors. An amount of each of the orthogonal column signals present on each of the plurality of row conductors is detected. The detected amount of each of the plurality of orthogonal row signals and the detected amount of each of the plurality of orthogonal column signals is used to determine the location of a touch event on the device.

This application includes material which is subject to copyrightprotection. The copyright owner has no objection to the facsimilereproduction by anyone of the patent disclosure, as it appears in thePatent and Trademark Office files or records, but otherwise reserves allcopyright rights whatsoever.

FIELD OF THE INVENTION

The disclosed system and method relate in general to the field of userinput, and in particular to user input systems which provide noisereduction in a fast multi-touch sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

Objects, features, and advantages of the invention will be apparent fromthe following more particular description of preferred embodiments asillustrated in the accompanying drawings, in which reference charactersrefer to the same parts throughout the various views. The drawings arenot necessarily to scale, emphasis instead being placed uponillustrating principles of the invention. Although example embodimentsand associated data are disclosed for the purpose of illustrating theinvention, other embodiments and associated data will be apparent to aperson of skill in the art, in view of this disclosure, withoutdeparting from the scope and spirit of the disclosure herein.

FIG. 1 provides a high level block diagram illustrating an embodiment ofa low-latency touch sensor device.

FIG. 2 illustrates an embodiment of a layout for crossing conductivepaths that can be used in an embodiment of a low-latency touch sensordevice.

FIG. 3 shows a block diagram illustrating a field flattening procedure.

FIG. 4 shows a diagram illustrating a four-connected neighborhood arounda local maximum.

FIG. 5 shows a diagram illustrating an eight-connected neighborhoodaround a local maximum.

FIG. 6 shows a geometric view illustrating an elliptical fit to anasymmetric touch point.

FIG. 7 provides a high level block diagram illustrating an embodiment ofa low-latency touch sensor device configured for noise reduction.

FIGS. 8-11, 12A and 12B are simplified diagrammatic illustrations ofsignal generation and transmission schemes.

FIG. 13 shows a side view illustrating a user identification techniqueaccording to an embodiment of the disclosed system and method.

FIGS. 14 and 15 show perspective views illustrating fast multi-touchstyli according to embodiments of the disclosed system and method.

FIG. 16 shows a top view illustrating a sensor sheet and an activeoptical stylus.

FIG. 17 shows a side view illustrating a sensor sheet and an activeoptical stylus.

FIG. 18 shows a side view illustrating internal reflection in a sensorsheet in accordance with an embodiment of the disclosed active opticalstylus.

FIG. 19 shows a side view illustrating use of an angular filter inaccordance with an embodiment of the disclosed active optical stylus.

FIG. 20 shows a side view illustrating patterns emitted onto a sensorsheet by an active optical stylus.

FIGS. 21-24 illustrate geometric projections of spots emitted by anactive optical stylus along the edges of a sensor sheet.

DETAILED DESCRIPTION

This application relates to user interfaces such as the fast multi-touchsensors and other interfaces disclosed in U.S. patent application Ser.No. 13/841,436 filed Mar. 15, 2013 entitled “Low-Latency Touch SensitiveDevice,” U.S. Patent Application No. 61/928,069 filed Jan. 16, 2014entitled “Fast Multi-Touch Update Rate Throttling,” U.S. patentapplication Ser. No. 14/046,819 filed Oct. 4, 2013 entitled “HybridSystems And Methods For Low-Latency User Input Processing And Feedback,”U.S. Patent Application No. 61/798,948 filed Mar. 15, 2013 entitled“Fast Multi-Touch Stylus,” U.S. Patent Application No. 61/799,035 filedMar. 15, 2013 entitled “Fast Multi-Touch Sensor With User-IdentificationTechniques,” U.S. Patent Application No. 61/798,828 filed Mar. 15, 2013entitled “Fast Multi-Touch Noise Reduction,” U.S. Patent Application No.61/798,708 filed Mar. 15, 2013 entitled “Active Optical Stylus,” U.S.Patent Application No. 61/710,256 filed Oct. 5, 2012 entitled “HybridSystems And Methods For Low-Latency User Input Processing And Feedback,”U.S. Patent Application No. 61/845,892 filed Jul. 12, 2013 entitled“Fast Multi-Touch Post Processing,” U.S. Patent Application No.61/845,879 filed Jul. 12, 2013 entitled “Reducing Control ResponseLatency With Defined Cross-Control Behavior,” U.S. Patent ApplicationNo. 61/879,245 filed Sep. 18, 2013 entitled “Systems And Methods ForProviding Response To User Input Using Information About State ChangesAnd Predicting Future User Input,” U.S. Patent Application No.61/880,887 filed Sep. 21, 2013 entitled “Systems And Methods ForProviding Response To User Input Using Information About State ChangesAnd Predicting Future User Input,” U.S. patent application Ser. No.14/046,823 filed Oct. 4, 2013 entitled “Hybrid Systems And Methods ForLow-Latency User Input Processing And Feedback,” U.S. patent applicationSer. No. 14/069,609 filed Nov. 1, 2013 entitled “Fast Multi-Touch PostProcessing,” U.S. Patent Application No. 61/887,615 filed Oct. 7, 2013entitled “Touch And Stylus Latency Testing Apparatus,” and U.S. PatentApplication No. 61/930,159 filed Jan. 22, 2014 entitled “DynamicAssignment Of Possible Channels In A Touch Sensor,” and U.S. PatentApplication No. 61/932,047 filed Jan. 27, 2014 entitled “DecimationStrategies For Input Event Processing.” The entire disclosures of thoseapplications are incorporated herein by this reference.

This disclosure will first describe the operation of fast multi-touchsensors to which the present post processing may be described. Thepresently disclosed system and method, however, is not limited to postprocessing in connection with the fast multi-touch sensors describedbelow, but rather may be applied broadly to other sensors withoutdeparting from the spirit and scope of the invention.

In an embodiment, a fast multi-touch sensor, which utilizes the postprocessing techniques disclosed herein, provides detection of touchevents (or other gestures) from human fingers (or other objects) on atwo-dimensional manifold and has the capability for a touch event, ormultiple simultaneous touch events, to be detected and distinguishedfrom each other. (As used herein, the phrase “touch event” and the word“touch” when used as a noun include a near touch and a near touch event,or any other gesture that can be identified using a sensor.) Inaccordance with an embodiment, touch events may be detected, processedand supplied to downstream computational processes with very lowlatency, e.g., on the order of ten milliseconds or less, or on the orderof less than one millisecond.

In an embodiment, the disclosed fast multi-touch sensor utilizes aprojected capacitive method that has been enhanced for high update rateand low latency measurements of touch events. The technique can useparallel hardware and higher frequency waveforms to gain the aboveadvantages. Also disclosed are methods to make sensitive and robustmeasurements, which methods may be used on transparent display surfacesand which may permit economical manufacturing of products which employthe technique. In this regard, a “capacitive object” as used hereincould be a finger, other part of the human body, a stylus, or any objectto which the sensor is sensitive. The sensors and methods disclosedherein need not rely on capacitance. With respect to the optical sensorembodiment disclosed below, such embodiments utilize photon tunnelingand leaking to sense a touch event, and a “capacitive object” as usedherein includes any object, such as a stylus or finger, that that iscompatible with such sensing. Similarly, “touch locations” and “touchsensitive device” as used herein do not require actual touching contactbetween a capacitive object and the disclosed sensor.

FIG. 1 illustrates certain principles of a fast multi-touch sensor 100in accordance with an embodiment. At reference no. 200, a differentsignal is transmitted into each of the surface's rows. The signals aredesigned to be “orthogonal”, i.e. separable and distinguishable fromeach other. At reference no. 300, a receiver is attached to each column.The receiver is designed to receive any of the transmitted signals, oran arbitrary combination of them, and to individually measure thequantity of each of the orthogonal transmitted signals present on thatcolumn. The touch surface 400 of the sensor comprises a series of rowsand columns (not all shown), along which the orthogonal signals canpropagate. In an embodiment, the rows and columns are designed so that,when they are not subject to a touch event, a lower or negligible amountof signal is coupled between them, whereas, when they are subject to atouch event, a higher or non-negligible amount of signal is coupledbetween them. (In an embodiment, the opposite could hold—having thelesser amount of signal represent a touch event, and the greater amountof signal represent a lack of touch.) As discussed above, the touch, ortouch event does not require a physical touching, but rather an eventthat affects the level of coupled signal.

With continued reference to FIG. 1, in an embodiment, generally, thecapacitive result of a touch event in the proximity of both a row andcolumn may cause a non-negligible amount of signal present on the row tobe coupled to the column. More generally, touch events cause, and thuscorrespond to, the received signals on the columns. Because the signalson the rows are orthogonal, multiple row signals can be coupled to acolumn and distinguished by the receiver. Likewise, the signals on eachrow can be coupled to multiple columns. For each column coupled to agiven row, the signals found on the column contain information that willindicate which rows are being touched simultaneously with that column.The quantity of each signal received is generally related to the amountof coupling between the column and the row carrying the correspondingsignal, and thus, may indicate a distance of the touching object to thesurface, an area of the surface covered by the touch and/or the pressureof the touch.

When a row and column are touched simultaneously, some of the signalthat is present on the row is coupled into the corresponding column. (Asdiscussed above, the term touch or touched does not require actualphysical contact, but rather, relative proximity.) Indeed, in variousimplementations of a touch device, physical contact with the rows and/orcolumns is unlikely as there may be a protective barrier between therows and/or columns and the finger or other object of touch. Moreover,generally, the rows and columns themselves are not in touch with eachother, but rather, placed in a proximity that prevents more than anegligible amount of signal to be coupled there-between. Generally, therow-column coupling results not from actual contact between them, nor byactual contact from the finger or other object of touch, but rather, bythe capacitive effect of bringing the finger (or other object) intoclose proximity—which close proximity resulting in capacitive effect isreferred to herein as touch.)

The nature of the rows and columns is arbitrary and the particularorientation is irrelevant. Indeed, the terms row and column are notintended to refer to a square grid, but rather to a set of conductorsupon which signal is transmitted (rows) and a set of conductors ontowhich signal may be coupled (columns). It is not even necessary that therows and columns be in a grid at all. Other shapes are possible as longas a touch event will touch part of a “row” and part of a “column”, andcause some form of coupling. For example, the “rows” could be inconcentric circles and the “columns” could be spokes radiating out fromthe center. Moreover, it is not necessary for there to be only two typessignal propagation channels: instead of rows and columns, in anembodiment, channels “A”, “B” and “C” may be provided, where signalstransmitted on “A” could be received on “B” and “C”, or, in anembodiment, signals transmitted on “A” and “B” could be received on “C”.It is also possible that the signal propagation channels can alternatefunction, sometimes supporting transmitters and sometimes supportingreceivers. Many alternative embodiments are possible and will beapparent to a person of skill in the art after considering thisdisclosure.

As noted above, in an embodiment the touch surface 400 comprises of aseries of rows and columns, along which signals can propagate. Asdiscussed above, the rows and columns are designed so that, when theyare not being touched, a negligible amount of signal is coupled betweenthem. Moreover, a different signal is transmitted into each of the rows.In an embodiment, each of these different signals are orthogonal (i.e.separable and distinguishable) from one another. When a row and columnare touched simultaneously, a non-negligible amount of the signal thatis present on the row is coupled into the corresponding column. Thequantity of the signal that is coupled onto a column may be related tothe pressure or area of touch.

A receiver 300 is attached to each column. The receiver is designed toreceive non-negligible amounts of any of the orthogonal signals, or anarbitrary combination of the orthogonal signals, and to identify thecolumns providing non-negligible amounts of signal. In an embodiment,the receiver may measure the quantity of each of the orthogonaltransmitted signals present on that column. In this manner, in additionto identifying the rows in touch with each column, the receiver canprovide additional (e.g., qualitative) information concerning the touch.In general, touch events may correspond to the received signals on thecolumns. For each column, the different signals received thereonindicate which of the corresponding rows is being touched simultaneouslywith that column. In an embodiment, the non-negligible quantity of eachsignal received may be related to the amount of coupling between thecorresponding row and column and may indicate the area of the surfacecovered by the touch, the pressure of the touch, etc.

Simple Sinusoid Embodiment

In an embodiment, the orthogonal signals being transmitted into the rowsmay be unmodulated sinusoids, each having a different frequency, thefrequencies being chosen so that they can be easily distinguished fromeach other in the receiver. In an embodiment, frequencies are selectedto provide sufficient spacing between them such that they can be easilydistinguished from each other in the receiver. In an embodiment, nosimple harmonic relationships exist between the selected frequencies.The lack of simple harmonic relationships may mitigate non-linearartifacts that can cause one signal to mimic another.

Generally, a “comb” of frequencies, where the spacing between adjacentfrequencies is constant, and the highest frequency is less than twicethe lowest, will meet these criteria if the spacing between frequencies,Δf, is at least the reciprocal of the measurement period For example, ifit is desired to measure a combination of signals (from a column, forexample) to determine which row signals are present once per millisecond(τ), then the frequency spacing (Δf) must be greater than one kilohertz(i.e., Δf>1/τ). According to this calculation, in an example case withonly ten rows, one could use the following frequencies:

Row 1: 5.000 MHz Row 2: 5.001 MHz Row 3: 5.002 MHz Row 4: 5.003 MHz Row5: 5.004 MHz Row 6: 5.005 MHz Row 7: 5.006 MHz Row 8: 5.007 MHz Row 9:5.008 MHz Row 10: 5.009 MHz

It will be apparent to one of skill in the art that frequency spacingmay be substantially greater than this minimum to permit robust design.As an example, a 20 cm by 20 cm touch surface with 0.5 cm row/columnspacing would require forty rows and forty columns and necessitatesinusoids at forty different frequencies. While a once per millisecondanalysis rate would require only 1 KHz spacing, an arbitrarily largerspacing is utilized for a more robust implementation. The arbitrarilylarger spacing is subject to the constraint that the maximum frequencyshould not be more than twice the lowest (i.e. f_(max)<2(f_(min))). Inthis example, a frequency spacing of 100 kHz with the lowest frequencyset at 5 MHz may be used, yielding a frequency list of 5.0 MHz, 5.1 MHz,5.2 MHz, etc. up to 8.9 MHz.

In an embodiment, each of the sinusoids on the list may be generated bya signal generator and transmitted on a separate row by the transmitter.To identify the rows and columns that are being simultaneously touched,a receiver receives any signals present on the columns and a signalprocessor analyzes the signal to determine which, if any, frequencies onthe list appear. In an embodiment, the identification can be supportedwith a frequency analysis technique (e.g., Fourier transform), or byusing a filter bank.

In an embodiment, from each column's signal, the receiver can determinethe strength of each frequency from the list of frequencies found in thesignal on that column. In an embodiment, where the strength of afrequency is greater than some threshold, the signal processoridentifies there being a touch event between the column and the rowcorresponding to that frequency. In an embodiment, signal strengthinformation, which may correspond to various physical phenomenaincluding the distance of the touch from the row/column intersection,the size of the touch object, the pressure with which the object ispressing down, the fraction of row/column intersection that is beingtouched, etc. may be used as an aid to localize the area of the touchevent.

Once the signals strengths have been calculated for at least twofrequencies (corresponding to rows) or for at least two columns, atwo-dimensional map can be created, with the signal strength being thevalue of the map at that row/column intersection. In an embodiment, thesignals' strengths are calculated for each frequency on each column.Once signal strengths are calculated a two-dimensional map may becreated. In an embodiment, the signal strength is the value of the mapat that row/column intersection. In an embodiment, due to physicaldifferences in the touch surface at different frequencies, the signalstrengths need to be normalized for a given touch or calibrated.Similarly, in an embodiment, due to physical differences across thetouch surface or between the intersections, the signal strengths need tobe normalized for a given touch or calibrated.

In an embodiment, the two-dimensional map data may be thresholded tobetter identify, determine or isolate touch events. In an embodiment,the two-dimensional map data may be used to infer information about theshape, orientation, etc. of the object touching the surface.

Returning to the discussion of the signals being transmitted on therows, a sinusoid is not the only orthogonal signal that can be used inthe configuration described above. Indeed, as discussed above, any setof signals that can be distinguished from each other will work.Nonetheless, sinusoids may have some advantageous properties that maypermit simpler engineering and more cost efficient manufacture ofdevices which use this technique. For example, sinusoids have a verynarrow frequency profile (by definition), and need not extend down tolow frequencies, near DC. Moreover, sinusoids can be relativelyunaffected by 1/f noise, which noise could affect broader signals thatextend to lower frequencies.

In an embodiment, sinusoids may be detected by a filter bank. In anembodiment, sinusoids may be detected by frequency analysis techniques(e.g., Fourier transform). Frequency analysis techniques may beimplemented in a relatively efficient manner and may tend to have gooddynamic range characteristics, allowing them to detect and distinguishbetween a large number of simultaneous sinusoids. In broad signalprocessing terms, the receiver's decoding of multiple sinusoids may bethought of as a form of frequency-division multiplexing. In anembodiment, other modulation techniques such as time-division andcode-division multiplexing could also be used. Time divisionmultiplexing has good dynamic range characteristics, but typicallyrequires that a finite time be expended transmitting into (or analyzingreceived signals from) the touch surface. Code division multiplexing hasthe same simultaneous nature as frequency-division multiplexing, but mayencounter dynamic range problems and may not distinguish as easilybetween multiple simultaneous signals.

Modulated Sinusoid Embodiment

In an embodiment, a modulated sinusoid may be used in lieu of, incombination with and/or as an enhancement of, the sinusoid embodimentdescribed above. The use of unmodulated sinusoids may causeradiofrequency interference to other devices near the touch surface, andthus, a device employing them might encounter problems passingregulatory testing (e.g., FCC, CE). In addition, the use of unmodulatedsinusoids may be susceptible to interference from other sinusoids in theenvironment, whether from deliberate transmitters or from otherinterfering devices (perhaps even another identical touch surface). Inan embodiment, such interference may cause false or degraded touchmeasurements in the described device.

In an embodiment, to avoid interference, the sinusoids may be modulatedor “stirred” prior to being transmitted by the transmitter in a mannerthat the signals can be demodulated (“unstirred”) once they reach thereceiver. In an embodiment, an invertible transformation (or nearlyinvertible transformation) may be used to modulate the signals such thatthe transformation can be compensated for and the signals substantiallyrestored once they reach the receiver. As will also be apparent to oneof skill in the art, signals emitted or received using a modulationtechnique in a touch device as described herein will be less correlatedwith other things, and thus, act more like mere noise, rather thanappearing to be similar to, and/or being subject to interference from,other signals present in the environment.

In an embodiment, a modulation technique utilized will cause thetransmitted data to appear fairly random or, at least, unusual in theenvironment of the device operation. Two modulation schemes arediscussed below: Frequency Modulation and Direct Sequence SpreadSpectrum Modulation.

Frequency Modulation

Frequency modulation of the entire set of sinusoids keeps them fromappearing at the same frequencies by “smearing them out.” Becauseregulatory testing is generally concerned with fixed frequencies,transmitted sinusoids that are frequency modulated will appear at loweramplitudes, and thus be less likely to be a concern. Because thereceiver will “un-smear” any sinusoid input to it, in an equal andopposite fashion, the deliberately modulated, transmitted sinusoids canbe demodulated and will thereafter appear substantially as they didprior to modulation. Any fixed frequency sinusoids that enter (e.g.,interfere) from the environment, however, will be “smeared” by the“unsmearing” operation, and thus, will have a reduced or an eliminatedeffect on the intended signal. Accordingly, interference that mightotherwise be caused to the sensor is lessened by employing frequencymodulation, e.g., to a comb of frequencies that, in an embodiment, areused in the touch sensor.

In an embodiment, the entire set of sinusoids may be frequency modulatedby generating them all from a single reference frequency that is,itself, modulated. For example, a set of sinusoids with 100 kHz spacingcan be generated by multiplying the same 100 kHz reference frequency bydifferent integers. In an embodiment, this technique can be accomplishedusing phase-locked loops. To generate the first 5.0 MHz sinusoid, onecould multiply the reference by 50, to generate the 5.1 MHz sinusoid,one could multiply the reference by 51, and so forth. The receiver canuse the same modulated reference to perform the detection anddemodulation functions. Direct Sequence Spread Spectrum Modulation

In an embodiment, the sinusoids may be modulated by periodicallyinverting them on a pseudo-random (or even truly random) schedule knownto both the transmitter and receiver. Thus, in an embodiment, beforeeach sinusoid is transmitted to its corresponding row, it is passedthrough a selectable inverter circuit, the output of which is the inputsignal multiplied by +1 or −1 depending on the state of an “invertselection” input. In an embodiment, all of these “invert selection”inputs are driven from the same signal, so that the sinusoids for eachrow are all multiplied by either +1 or −1 at the same time. In anembodiment, the signal that drives the “invert selection” input may be apseudorandom function that is independent of any signals or functionsthat might be present in the environment. The pseudorandom inversion ofthe sinusoids spreads them out in frequency, causing them to appear likerandom noise so that they interfere negligibly with any devices withwhich they might come in contact.

On the receiver side, the signals from the columns may be passed throughselectable inverter circuits that are driven by the same pseudorandomsignal as the ones on the rows. The result is that, even though thetransmitted signals have been spread in frequency, they are despreadbefore the receiver because they have been ben multiplied by either +1or −1 twice, leaving them in, or returning them to, their unmodifiedstate. Applying direct sequence spread spectrum modulation may spreadout any interfering signals present on the columns so that they act onlyas noise and do not mimic any of the set of intentional sinusoids.

In an embodiment, selectable inverters can be created from a smallnumber of simple components and/or can be implemented in transistors ina VLSI process.

Because many modulation techniques are independent of each other, in anembodiment, multiple modulation techniques could be employed at the sametime, e.g. frequency modulation and direct sequence spread spectrummodulation of the sinusoid set. Although potentially more complicated toimplement, such multiple modulated implementation may achieve betterinterference resistance.

Because it would be extremely rare to encounter a particular pseudorandom modulation in the environment, it is likely that the multi-touchsensors described herein would not require a truly random modulationschedule. One exception may be where more than one touch surface withthe same implementation is being touched by the same person. In such acase, it may be possible for the surfaces to interfere with each other,even if they use very complicated pseudo random schedules. Thus, in anembodiment, care is taken to design pseudo random schedules that areunlikely to conflict. In an embodiment, some true randomness may beintroduced into the modulation schedule. In an embodiment, randomness isintroduced by seeding the pseudo random generator from a truly randomsource and ensuring that it has a sufficiently long output duration(before it repeats). Such an embodiment makes it highly unlikely thattwo touch surfaces will ever be using the same portion of the sequenceat the same time. In an embodiment, randomness is introduced byexclusive or'ing (XOR) the pseudo random sequence with a truly randomsequence. The XOR function combines the entropy of its inputs, so thatthe entropy of its output is never less than either input.

A Low-Cost Implementation Embodiment

Touch surfaces using the previously described techniques may have arelatively high cost associated with generating and detecting sinusoidscompared to other methods. Below are discussed methods of generating anddetecting sinusoids that may be more cost-effective and/or be moresuitable for mass production.

Sinusoid Detection

In an embodiment, sinusoids may be detected in a receiver using acomplete radio receiver with a Fourier Transform detection scheme. Suchdetection may require digitizing a high-speed RF waveform and performingdigital signal processing thereupon. Separate digitization and signalprocessing may be implemented for every column of the surface; thispermits the signal processor to discover which of the row signals are intouch with that column. In the above-noted example, having a touchsurface with forty rows and forty columns, would require forty copies ofthis signal chain. Today, digitization and digital signal processing arerelatively expensive operations, in terms of hardware, cost, and power.It would be useful to utilize a more cost-effective method of detectingsinusoids, especially one that could be easily replicated and requiresvery little power.

In an embodiment, sinusoids may be detected using a filter bank. Afilter bank comprises an array of bandpass filters that can take aninput signal and break it up into the frequency components associatedwith each filter. The Discrete Fourier Transform (DFT, of which the FFTis an efficient implementation) is a form of a filter bank withevenly-spaced bandpass filters that is commonly used for frequencyanalysis. DFTs may be implemented digitally, but the digitization stepmay be expensive. It is possible to implement a filter bank out ofindividual filters, such as passive LC (inductor and capacitor) or RCactive filters. Inductors are difficult to implement well on VLSIprocesses, and discrete inductors are large and expensive, so it may notbe cost effective to use inductors in the filter bank.

At lower frequencies (about 10 MHz and below), it is possible to buildbanks of RC active filters on VLSI. Such active filters may performwell, but may also take up a lot of die space and require more powerthan is desirable.

At higher frequencies, it is possible to build filter banks with surfaceacoustic wave (SAW) filter techniques. These allow nearly arbitrary FIRfilter geometries. SAW filter techniques require piezoelectric materialswhich are more expensive than straight CMOS VLSI. Moreover, SAW filtertechniques may not allow enough simultaneous taps to integratesufficiently many filters into a single package, thereby raising themanufacturing cost.

In an embodiment, sinusoids may be detected using an analog filter bankimplemented with switched capacitor techniques on standard CMOS VLSIprocesses that employs an FFT-like “butterfly” topology. The die arearequired for such an implementation is typically a function of thesquare of the number of channels, meaning that a 64-channel filter bankusing the same technology would require only 1/256th of the die area ofthe 1024-channel version. In an embodiment, the complete receive systemfor the low-latency touch sensor is implemented on a plurality of VLSIdies, including an appropriate set of filter banks and the appropriateamplifiers, switches, energy detectors, etc. In an embodiment, thecomplete receive system for the low-latency touch sensor is implementedon a single VLSI die, including an appropriate set of filter banks andthe appropriate amplifiers, switches, energy detectors, etc. In anembodiment, the complete receive system for the low-latency touch sensoris implemented on a single VLSI die containing n instances of ann-channel filter bank, and leaving room for the appropriate amplifiers,switches, energy detectors, etc.

Sinusoid Generation

Generating the transmit signals (e.g., sinusoids) in a low-latency touchsensor is generally less complex than detection, principally becauseeach row requires the generation of a single signal while the columnreceivers have to detect and distinguish between many signals. In anembodiment, sinusoids can be generated with a series of phase-lockedloops (PLLs), each of which multiply a common reference frequency by adifferent multiple.

In an embodiment, the low-latency touch sensor design does not requirethat the transmitted sinusoids are of very high quality, but rather,accommodates transmitted sinusoids that have more phase noise, frequencyvariation (over time, temperature, etc.), harmonic distortion and otherimperfections than may usually be allowable or desirable in radiocircuits. In an embodiment, the large number of frequencies may begenerated by digital means and then employ a relatively coarseanalog-to-digital conversion process. As discussed above, in anembodiment, the generated row frequencies should have no simple harmonicrelationships with each other, any non-linearities in the describedgeneration process should not cause one signal in the set to “alias” ormimic another.

In an embodiment, a frequency comb may be generated by having a train ofnarrow pulses filtered by a filter bank, each filter in the bankoutputting the signals for transmission on a row. The frequency “comb”is produced by a filter bank that may be identical to a filter bank thatcan be used by the receiver. As an example, in an embodiment, a 10nanosecond pulse repeated at a rate of 100 kHz is passed into the filterbank that is designed to separate a comb of frequency componentsstarting at 5 MHz, and separated by 100 kHz. The pulse train as definedwould have frequency components from 100 kHz through the tens of MHz,and thus, would have a signal for every row in the transmitter. Thus, ifthe pulse train were passed through an identical filter bank to the onedescribed above to detect sinusoids in the received column signals, thenthe filter bank outputs will each contain a single sinusoid that can betransmitted onto a row.

Transparent Display Surface

It may be desirable that the touch surface be integrated with a computerdisplay so that a person can interact with computer-generated graphicsand imagery. While front projection can be used with opaque touchsurfaces and rear projection can be used with translucent ones, modernflat panel displays (LCD, plasma, OLED, etc.) generally require that thetouch surface be transparent. In an embodiment, the present technique'srows and columns, which allow signals to propagate along them, need tobe conductive to those signals. In an embodiment, the presenttechnique's rows and columns, which allow radio frequency signals topropagate along them, need to be electrically conductive.

If the rows and columns are insufficiently conductive, the resistanceper unit length along the row/column will combine with the capacitanceper unit length to form a low-pass filter: any high-frequency signalsapplied at one end will be substantially attenuated as they propagatealong the poor conductor.

Visually transparent conductors are commercially available (e.g.indium-tin-oxide or ITO), but the tradeoff between transparency andconductivity is problematic at the frequencies that may be desirable forsome embodiments of the low-latency touch sensor described herein: ifthe ITO were thick enough to support certain desirable frequencies overcertain lengths, it may be insufficiently transparent for someapplications. In an embodiment, the rows and/or columns may be formedentirely, or at least partially, from graphene and/or carbon nanotubes,which are both highly conductive and optically transparent.

In an embodiment, the rows and/or columns may be formed from one or morefine wires that block a negligible amount of the display behind them. Inan embodiment, the fine wires are too small to see, or at least toosmall to present a visual impediment when viewing a display behind it.In an embodiment, fine silver wires patterned onto transparent glass orplastic can be used to make up the rows and/or columns. Such fine wiresneed to have sufficient cross section to create a good conductor alongthe row/column, but it is desirable (for rear displays) that such wiresare small enough and diffuse enough to block as little of the underlyingdisplay as appropriate for the application. In an embodiment, the finewire size is selected on the basis of the pixels size and/or pitch ofthe underlying display.

As an example, the new Apple Retina displays comprises about 300 pixelsper inch, which yields a pixel size of about 80 microns on a side. In anembodiment, a 20 micron diameter silver wire 20 centimeters long (thelength of an iPad display), which has a resistance of about 10 ohms, isused as a row and/or column and/or as part of a row and/or column in alow-latency touch sensor as described herein. Such 20 micron diametersilver wire, however, if stretched across a retina display, may block upto 25% of an entire line of pixels. Accordingly, in an embodiment,multiple thinner diameter silver wires may be employed as a column orrow, which can maintain an appropriate resistance, and provideacceptable response with respect to radiofrequency skin depth issues.Such multiple thinner diameter silver wires can be laid in a patternthat is not straight, but rather, somewhat irregular. A random orirregular pattern of thinner wires is likely to be less visuallyintrusive. In an embodiment, a mesh of thin wires is used; the use of amesh will improve robustness, including against manufacturing flaws inpatterning. In an embodiment, single thinner diameter wires may beemployed as a column or row, provided that the thinner wire issufficiently conductive to maintain an appropriate level resistance, andacceptable response with respect to radiofrequency skin depth issues.

FIG. 2 illustrates an embodiment of a row/column touch surface that hasa diamond-shaped row/column mesh. This mesh pattern is designed toprovide maximal and equal surface area to the rows and columns whilepermitting minimal overlap between them.

A touch event with an area greater than one of the diamonds will coverat least part of a row and a column, which will permit some coupling ofa row signal into the overlapped column. In an embodiment, the diamondsare sized to be smaller than the touching implement (finger, stylus,etc.). In an embodiment, a 0.5 cm spacing between rows and columnsperforms well for human fingers.

In an embodiment, a simple grid of wires is employed as the rows andcolumns. Such a grid would provide less surface area for the rows andcolumns, but can suffice for radio frequency signals, and provide asufficient non-negligible coupling which can be detected by a receiver.

In an embodiment, the “diamond patterns” for the rows and columns, asshown in FIG. 2, can be created by using a randomly connected mesh ofthin wires that fills the space of the indicated shapes, or by combiningwire mesh and an another transparent conductor such as ITO. In anembodiment, thin wires may be used for long stretches of conductivity,e.g., across the entire screen, and ITO may be used for local areas ofconductivity, such as the diamond-shaped areas.

An Optical Embodiment

While radiofrequency and electrical methods of implementing thedescribed fast multi-touch technique have been discussed above, othermedia can be employed as well. For example, the signals can be opticalsignals (i.e., light), having waveguides or other means for the rows andcolumns. In an embodiment, the light, used for the optical signals maybe in the visible region, the infrared and/or the ultraviolet.

In an embodiment, instead of electrically conductive rows and columnsthat carry radiofrequency signals, the rows and columns could compriseoptical waveguides, such as optical fiber, fed by one or more lightsources that generate orthogonal signals and are coupled to thewaveguides by an optical coupler. For example, a different distinctwavelength of light could be injected into each row fiber. When a humanfinger touches a row fiber, some of the light in it will leak (i.e.,couple) into the finger, due to frustrated total internal reflection.Light from the finger may then enter one of the column fibers, due tothe reciprocal process, and propagate to a detector at the end of thefiber.

In an embodiment, optical signals may be generated with LEDs ofdifferent wavelengths, or by using optical filters. In an embodiment,custom interference filters are employed. In an embodiment, thedifferent wavelengths of light present on the fiber columns can bedetected using optical filter banks. In an embodiment, such opticalfilter banks may be implemented using custom interference filters. In anembodiment, wavelengths of light outside the visible spectrum (e.g.,infrared and/or ultraviolet light) may be used to avoid adding extravisible light to the display.

In an embodiment, the row and column fibers may be woven together sothat a finger can touch them simultaneously. In an embodiment, the wovenconstruction may be made as visually transparent as needed to avoidobscuring the display.

Fast Multi-Touch Post Processing

After the signal strengths from each row in each column have beencalculated using, for example, the procedures described above,post-processing is performed to convert the resulting 2-D “heat map”into usable touch events. In an embodiment, such post processingincludes at least some of the following four procedures: fieldflattening, touch point detection, interpolation and touch pointmatching between frames. The field flattening procedure subtracts anoffset level to remove crosstalk between rows and columns, andcompensates for differences in amplitude between particular row/columncombinations due to attenuation. The touch point detection procedurecomputes the coarse touch points by finding local maxima in theflattened signal. The interpolation procedure computes the fine touchpoints by fitting data associated with the coarse touch points to aparaboloid. The frame matching procedure matches the calculated touchpoints to each other across frames. Below, each of the four proceduresis described in turn. Also disclosed are examples of implementation,possible failure modes, and consequences, for each processing step.Because of the requirement for very low latency, the processing stepsshould be optimized and parallelized.

We first describe the field flattening procedure. Systematic issues dueto the design of the touch surface and sensor electronics may causeartifacts in each column's received signal strength. These artifacts canbe compensated-for as follows. First, because of cross-talk between therows and columns, the received signal strength for each row/columncombination will experience an offset level. To a good approximation,this offset level will be constant and can be subtracted off.

Second, the amplitude of the signal received at a column due to acalibrated touch at a given row and column intersection will depend onthat particular row and column, mostly due to attenuation of the signalsas they propagate along the row and column. The farther they travel, themore attenuation there will be, so columns farther from the transmittersand rows farther from the receivers will have lower signal strengths inthe “heat map” than their counterparts. If the RF attenuation of therows and columns is low, the signal strength differences may benegligible and little or no compensation will be necessary. If theattenuation is high, compensation may be necessary or may improve thesensitivity or quality of touch detection. Generally, the signalstrengths measured at the receivers are expected to be linear with theamount of signal transmitted into the columns. Thus, in an embodiment,compensation will involve multiplying each location in the heat map by acalibration constant for that particular row/column combination. In anembodiment, measurements or estimates may be used to determine a heatmap compensation table, which table can be similarly used to provide thecompensation by multiplication. In an embodiment, a calibrationoperation is used to create a heat map compensation table. The term“heat map” as used herein need not require an actual map of heat, butrather the term can mean any array of at least two dimensions comprisingdata corresponding to locations.

In an exemplary embodiment, the entire field flattening procedure is asfollows. With nothing touching the surface, first measure the signalstrength for each row signal at each column receiver. Because there areno touches, substantially the entire signal received is due tocross-talk. The value measured (e.g., the amount of each row's signalfound on each column) is an offset level that needs to be subtractedfrom that position in the heat map. Then, with the constant offsetssubtracted, place a calibrated touch object at each row/columnintersection and measure the signal strength of that row's signal atthat column receiver. The signal processor may be configured tonormalize the touch events to the value of one location on the touchsurface. We can arbitrarily choose the location likely to have thestrongest signals (because it experiences the least attenuation), i.e.the row/column intersection closest to the transmitters and receivers.If the calibrated touch signal strength at this location is SN and thecalibrated touch signal strength for each row and column is S_(R,C)then, if we multiply each location in the heat map by (S_(N)/S_(R,c)),then all touch values will be normalized. For calibrated touches, thenormalized signal strength for any row/column in the heat map will beequal to one.

The field flattening procedure parallelizes well. Once the offsets andnormalization parameters are measured and stored—which should only needto be done once (or possibly again at a maintenance interval)—thecorrections can be applied as soon as each signal strength is measured.FIG. 3 illustrates an embodiment of a field flattening procedure.

In an embodiment, calibrating each row/column intersection may berequired at regular or selected maintenance intervals. In an embodiment,calibrating each row/column intersection may be required once per unit.In an embodiment, calibrating each row/column intersection may berequired once per design. In an embodiment, and particularly where,e.g., RF attenuation of the rows and columns is low, calibrating eachrow/column intersection may not be required at all. Moreover, in anembodiment where the signal attenuation along the rows and columns isfairly predictable, it may be possible to calibrate an entire surfacefrom only a few intersection measurements.

If a touch surface does experience a lot of attenuation, the fieldflattening procedure will, at least to some degree, normalize themeasurements, but it may have some side effects. For example, the noiseon each measurement will grow as its normalization constant gets larger.It will be apparent to one of skill in the art, that for lower signalstrengths and higher attenuations, this may cause errors and instabilityin the touch point detection and interpolation processes. Accordingly,in an embodiment, care is taken to provide sufficient signal strengthfor the largest attenuation (e.g., the farthest row/columnintersection).

We now turn to touch point detection. Once the heat map is generated andthe field flattened, one or more coarse touch points can be identified.Identifying the one or more coarse touch points is done by finding localmaxima in the normalized (i.e., flattened) signal strengths. A fast andparallelizable method for finding the one or more touch points involvescomparing each element of the normalized heat map to its neighbors andlabel it a local maximum if it is strictly greater than all of them. Inan embodiment, a point is identified as a local maximum if it is bothstrictly greater than all of its neighbors and above a given threshold.

It is within the scope of this disclosure to define the set of neighborsin various ways. In an embodiment, the nearest neighbors are defined bya Von Neumann neighborhood. In an embodiment, the nearest neighbors aredefined by a Moore neighborhood. The Von Neumann neighborhood mayconsist of the four elements that are vertically and horizontallyadjacent to the element in the center (i.e. the elements to the north,south, east and west of it). This is also called a “four-connected”neighborhood. More complex (i.e., larger) Von Neumann neighborhoods arealso applicable and may be used. The Moore neighborhood consists of theeight elements that are vertically, horizontally and diagonally adjacentto the element in the center (i.e. the elements to the north, south,east, west, northeast, northwest, southeast and southwest of it). Thisis also called the “eight-connected” neighborhood.

The neighborhood chosen will depend on the interpolation scheme used tocalculate the fine touch points. This is illustrated in further detailbelow.

In a given neighbor comparison, a special case may exist where anelement's normalized signal strength is equal to one or more of itsneighbors, strictly, or within a tolerance to allow for noise levels. Inan embodiment, neither point in such pairs is considered to be a touchpoint even if they have values above the threshold. In an embodiment,both points in such pairs are considered to be touch points. In anembodiment, regions where two or more neighboring points haveapproximately the same value are treated as one touch event. In anembodiment, regions where two or more neighboring points haveapproximately the same value are treated as a different type of touchevent (e.g., perhaps someone has their wrist in contact with the touchsurface) from the regions where a single local maxima can be found.

Turning now to the interpolation procedure. Once the coarse touch pointshave been determined (i.e., identified), fine touch points can becomputed using interpolation. In an embodiment, the capacitive contactof a distributed touch is fit to a model function having a maximum. Inan embodiment, the model function is a second-order function in two ormore dimensions. In an embodiment, the second-order function is aparaboloid. In an embodiment, the paraboloid model is an acceptableapproximation for a variety of objects that may be used to touch a touchsurface, such as a finger or stylus. Moreover, as discussed below, theparaboloid model is relatively non-intensive computationally. In anembodiment, a more complex or more computationally intensive model maybe used to provide more accurate estimation of the touch from theflattened heat map. For the purposes of the discussion below, theparaboloid is used as an illustrative example, but as will be apparentto one of skill in the art, other models, including models of greater orlesser complexity may be employed for the purpose of interpolation.

FIG. 4 illustrates a Von Neumann neighborhood around an exemplary localmaximum. For such a four-connected, or Von Neumann, neighborhood, therelevant points would look like those shown, with the central elementbeing the local maximum and the subscripts being the coordinates of aparticular element relative to it. The positions and signal strengths ofthe five elements allow us to fit them to the following equationdefining a paraboloid:

Ax ² +Cy ² +Dx+Ey+F=z

Where x and y are the position of an element, z is the signal strengthof the element, and A, C, D, E and F are the coefficients of thesecond-order polynomial. Relative to the central point, all of elementx, y positions are constant. The z values are the measured signalstrengths at each element, and thus are known. In an embodiment, fivesimultaneous equations can be used to solve for the five unknownpolynomial coefficients. Each equation represents one of the fivepoints, including the central point and its four neighbors.

In an embodiment, a Vandermonde-like matrix can be employed to solve forthe polynomial coefficients, as follows:

${\begin{bmatrix}x_{0,1}^{2} & y_{0,1}^{2} & x_{0,1} & y_{0,1} & 1 \\x_{{- 1},0}^{2} & y_{{- 1},0}^{2} & x_{{- 1},0} & y_{{- 1},0} & 1 \\x_{0,0}^{2} & y_{0,0}^{2} & x_{0,0} & y_{0,0} & 1 \\x_{1,0}^{2} & y_{1,0}^{2} & x_{1,0} & y_{1,0} & 1 \\x_{0,{- 1}}^{2} & y_{0,{- 1}}^{2} & x_{0,{- 1}} & y_{0,{- 1}} & 1\end{bmatrix}\begin{bmatrix}A \\C \\D \\E \\F\end{bmatrix}} = \begin{bmatrix}z_{0,1} \\z_{{- 1},0} \\z_{0,0} \\z_{1,0} \\z_{0,{- 1}}\end{bmatrix}$

Substituting in the values for the element positions, we get:

${\begin{bmatrix}0 & 1 & 0 & 1 & 1 \\1 & 0 & {- 1} & 0 & 1 \\0 & 0 & 0 & 0 & 1 \\1 & 0 & 1 & 0 & 1 \\0 & 1 & 0 & {- 1} & 1\end{bmatrix}\begin{bmatrix}A \\C \\D \\E \\F\end{bmatrix}} = \begin{bmatrix}z_{0,1} \\z_{{- 1},0} \\z_{0,0} \\z_{1,0} \\z_{0,{- 1}}\end{bmatrix}$

And then solve for the polynomial coefficients by inverting the constantVandermonde-like matrix:

$\begin{bmatrix}0 & 1 & 0 & 1 & 1 \\1 & 0 & {- 1} & 0 & 1 \\0 & 0 & 0 & 0 & 1 \\1 & 0 & 1 & 0 & 1 \\0 & 1 & 0 & {- 1} & 1\end{bmatrix}^{- 1} = {\frac{1}{2}\begin{bmatrix}0 & 1 & {- 2} & 1 & 0 \\1 & 0 & {- 2} & 0 & 1 \\0 & {- 1} & 0 & 1 & 0 \\1 & 0 & 0 & 0 & {- 1} \\0 & 0 & 2 & 0 & 0\end{bmatrix}}$

This yields:

$\begin{bmatrix}A \\C \\D \\E \\F\end{bmatrix} = {{\frac{1}{2}\begin{bmatrix}0 & 1 & {- 2} & 1 & 0 \\1 & 0 & {- 2} & 0 & 1 \\0 & {- 1} & 0 & 1 & 0 \\1 & 0 & 0 & 0 & {- 1} \\0 & 0 & 2 & 0 & 0\end{bmatrix}}\begin{bmatrix}z_{0,1} \\z_{{- 1},0} \\z_{0,0} \\z_{1,0} \\z_{0,{- 1}}\end{bmatrix}}$

In an embodiment, the polynomial coefficients are a linear combinationof the signal strengths and only simple multiplication, involvingnegation and a single shift, are required to calculate them;accordingly, they can be efficiently computed in an FPGA or ASIC.

At the maximum of the paraboloid, both partial derivatives are zero:

$\frac{\partial x}{\partial z} = {{{2{Ax}} + D} = {0\mspace{20mu} {and}}}$$\frac{\partial y}{\partial z} = {{{2{Cy}} + E} = 0}$

This will occur at the point x_(f), y_(f) where:

$x_{f} = {{{- \frac{D}{2A}}\mspace{14mu} {and}\mspace{14mu} y_{f}} = {- \frac{E}{2C}}}$

Thus, in an embodiment where the neighborhood data is fit to aparaboloid, and because a paraboloid has one maximum, that maximum isused as a location of the fine touch point. In an embodiment utilizingthe four-connected neighborhood, the values x_(f), and y_(f) areindependent of each other, with x_(f), depending only on the signalstrengths of the elements to the left and right of the center point, andy_(f) depending only on the signal strengths of the elements above andbelow it.

FIG. 5 illustrates a Moore or eight-connected neighborhood around alocal maximum. For such an eight-connected, or Moore, neighborhood, therelevant points would appear as shown, with the central element beingthe local maximum and the subscripts being the coordinates of aparticular element relative to it. The positions and signal strengths ofthe nine elements can be fit to a paraboloid equation. Because moreinput data is available in this example than the previous example, asomewhat more complex equation for a parabolid can be employed:

Ax ² +Bxy+Cy ² +Dx+Ey+F=z

This equation has an added xy cross term and a new B coefficient thatpermits the model to compensate for elongation in a direction other thanx or y. Again, relative to the central point, all of the element x, ypositions are constant and the z values are known. Nine simultaneousequations (one per element) can be used to determine (i.e.,overdetermine) the six unknown polynomial coefficients. A least-squarestechnique may be used to solve for the six unknown polynomialcoefficients.

A Vandermonde-like matrix may be used to fit the polynomial. Unlike theembodiment described above, the matrix is non-square, with nine rows andsix columns.

$\quad{{\begin{bmatrix}x_{{- 1},1}^{2} & {xy}_{{- 1},1} & y_{{- 1},1}^{2} & x_{{- 1},1} & y_{{- 1},1} & 1 \\x_{0,1}^{2} & {xy}_{0,1} & y_{0,1}^{2} & x_{0,1} & y_{0,1} & 1 \\x_{1,1}^{2} & {xy}_{1,1} & y_{1,1}^{2} & x_{1,1} & y_{1,1} & 1 \\x_{{- 1},0}^{2} & {xy}_{{- 1},0} & y_{{- 1},0}^{2} & x_{{- 1},0} & y_{{- 1},0} & 1 \\x_{0,0}^{2} & {xy}_{0,0} & y_{0,0}^{2} & x_{0,0} & y_{0,0} & 1 \\x_{1,0}^{2} & {xy}_{1,0} & y_{1,0}^{2} & x_{1,0} & y_{1,0} & 1 \\x_{{- 1},{- 1}}^{2} & {xy}_{{- 1},{- 1}} & y_{{- 1},{- 1}}^{2} & x_{{- 1},{- 1}} & y_{{- 1},{- 1}} & 1 \\x_{0,{- 1}}^{2} & {xy}_{0,{- 1}} & y_{0,{- 1}}^{2} & x_{0,{- 1}} & y_{0,{- 1}} & 1 \\x_{1,{- 1}}^{2} & {xy}_{1,{- 1}} & y_{1,{- 1}}^{2} & x_{1,{- 1}} & y_{1,{- 1}} & 1\end{bmatrix}\begin{bmatrix}A \\B \\C \\D \\E \\F\end{bmatrix}} = \begin{bmatrix}z_{{- 1},1} \\z_{0,1} \\z_{1,1} \\z_{{- 1},0} \\z_{0,0} \\z_{1,0} \\z_{{- 1},{- 1}} \\z_{0,{- 1}} \\z_{1,{- 1}}\end{bmatrix}}$

All of the entires in the Vandermonde-like matrix are constant, and thez values are known so, substituting in the constant values, yields

${\begin{bmatrix}1 & 1 & 1 & {- 1} & {- 1} & 1 \\0 & 0 & 1 & 0 & {- 1} & 1 \\1 & {- 1} & 1 & 1 & {- 1} & 1 \\1 & 0 & 0 & {- 1} & 0 & 1 \\0 & 0 & 0 & 0 & 0 & 1 \\1 & 0 & 0 & 1 & 0 & 1 \\1 & {- 1} & 1 & {- 1} & 1 & 1 \\0 & 0 & 1 & 0 & 1 & 1 \\1 & 1 & 1 & 1 & 1 & 1\end{bmatrix}\begin{bmatrix}A \\B \\C \\D \\E \\F\end{bmatrix}} = \begin{bmatrix}z_{{- 1},1} \\z_{0,1} \\z_{1,1} \\z_{{- 1},0} \\z_{0,0} \\z_{1,0} \\z_{{- 1},{- 1}} \\z_{0,{- 1}} \\z_{1,{- 1}}\end{bmatrix}$

Because the Vandermonde-like matrix is non-square, it cannot be invertedto solve for the polynomial coefficients. It can be solved, however,using its Moore-Penrose pseudo-inverse and performing a least squaresfit to the polynomial coefficients. In an embodiment, the pseudo inverseis defined as:

  pinv(X) = (X^(T)X)⁻¹X^(T) ${{pinv}\begin{bmatrix}1 & 1 & 1 & {- 1} & {- 1} & 1 \\0 & 0 & 1 & 0 & {- 1} & 1 \\1 & {- 1} & 1 & 1 & {- 1} & 1 \\1 & 0 & 0 & {- 1} & 0 & 1 \\0 & 0 & 0 & 0 & 0 & 1 \\1 & 0 & 0 & 1 & 0 & 1 \\1 & {- 1} & 1 & {- 1} & 1 & 1 \\0 & 0 & 1 & 0 & 1 & 1 \\1 & 1 & 1 & 1 & 1 & 1\end{bmatrix}} = {\frac{1}{36}\begin{bmatrix}6 & {- 12} & 6 & 6 & {- 12} & 6 & 6 & {- 12} & 6 \\{- 9} & 0 & 9 & 0 & 0 & 0 & 9 & 0 & {- 9} \\6 & 6 & 6 & {- 12} & {- 12} & {- 12} & 6 & 6 & 6 \\{- 6} & 0 & 6 & {- 6} & 0 & 6 & {- 6} & 0 & 6 \\6 & 6 & 6 & 0 & 0 & 0 & {- 6} & {- 6} & {- 6} \\{- 4} & 8 & {- 4} & 8 & 20 & 8 & {- 4} & 8 & {- 4}\end{bmatrix}}$

giving:

$\begin{bmatrix}A \\B \\C \\D \\E \\F\end{bmatrix} = {{\frac{1}{36}\begin{bmatrix}6 & {- 12} & 6 & 6 & {- 12} & 6 & 6 & {- 12} & 6 \\{- 9} & 0 & 9 & 0 & 0 & 0 & 9 & 0 & {- 9} \\6 & 6 & 6 & {- 12} & {- 12} & {- 12} & 6 & 6 & 6 \\{- 6} & 0 & 6 & {- 6} & 0 & 6 & {- 6} & 0 & 6 \\6 & 6 & 6 & 0 & 0 & 0 & {- 6} & {- 6} & {- 6} \\{- 4} & 8 & {- 4} & 8 & 20 & 8 & {- 4} & 8 & {- 4}\end{bmatrix}}\begin{bmatrix}z_{{- 1},1} \\z_{0,1} \\z_{1,1} \\z_{{- 1},0} \\z_{0,0} \\z_{1,0} \\z_{{- 1},{- 1}} \\z_{0,{- 1}} \\z_{1,{- 1}}\end{bmatrix}}$

The polynomial coefficients are a linear combination of the signalstrengths. The multiplications are slightly more complicated, but manyof the multiplicands can be factored out and applied a single time nearthe end of the calculation. The purpose of this step is to find themaximum of a paraboloid. Accordingly, overall scale factors areirrelevant, and focus need only be on relative values and argumentswhich maximize the function, in an embodiment, many of the operationsmay be able to cancel out, improving the efficiency of implementation.

As above, the fine touch point is presumed at the maximum of theparaboloid, where both partial derivatives are zero:

$\frac{\partial x}{\partial z} = {{{2{Ax}} + {By} + D} = {0\mspace{20mu} {and}}}$$\frac{\partial y}{\partial z} = {{{Bx} + {2{Cy}} + E} = 0}$

This will occur at the point x_(f), y_(f) where:

x _(f)=(BE−2CD)/(4AC−B ²) and y _(f)=(DB−2AE)/(4AC−B ²)

For the eight-connected neighborhood, the values x_(f) and y_(f) are notindependent of each other. Both depend on the signal strengths of alleight neighbors. Thus, this approach may have an increased computationalburden and the possibility that certain combinations of signal strengthswill produce singular values for the fine touch points. In an embodimentusing the least-squares approach on the eight Moore neighbors, such animplementation is more robust against noisy signal strength values. Inother words, in an embodiment, small errors in one signal strength willbe compensated for by the increased amount of data used in thecalculation, and the self-consistency of that data.

Moreover, the eight-connected neighborhood provides a B coefficient—anextra piece of information—that might prove useful as part of a userinterface. The B coefficient of the xy cross-term can be used tocharacterize asymmetry in the fitted paraboloid and, along with theaspect ratio information inherent in the A and C coefficients, whichcould allow software to determine the angle at which a touch isoccurring.

FIG. 6 shows an example touch point with an elliptical cross section,which could be obtained by truncating the paraboloid at a particular zvalue. The values of a and b can be obtained from the A and Ccoefficients of the polynomial, and they provide information about theaspect ratio of the object touching the surface. For example, a fingeror stylus would not necessarily be circularly symmetric, and the ratioof a to b could provide information about its shape.

Knowledge of the angle ϕ can provide information on the orientation ofthe ellipse, and might, for example, indicate which way a finger orstylus is pointing. ϕ can be calculated from the eigenvalues andeignevectors of the 2×2 matrix M given by the following:

$M = \begin{bmatrix}A & {B\text{/}2} \\{B\text{/}2} & C\end{bmatrix}$

This matrix will have two eignevalues and two eigenvectors. Theeigevector associated with the largest eigenvalue will point in thedirection of the ellipse's major axis. The other eigenvector will pointin the direction of the minor axis. The eigenvalues, λ₁ and λ₂ can becomputed as follows:

$\lambda_{i} = \frac{{{tr}(M)} \pm \sqrt{{{tr}(M)}^{2} - {4\; {\det (M)}}}}{2}$

Where tr(M) is the trace of the matrix M, which is equal to AC, anddet(M) is the determinant of the matrix M, which is equal to AC−B²/4.

Once the eigenvalues are obtained, we can use the Cayley-Hamiltontheorem to compute the eigenvectors. The eigenvector associated with λ₁is either of the columns of the matrix M −λ₂I and the eigenvectorassociated with λ₂ is either of the columns of the matrix M−λ₁I. Notethe reversal of the eigenvalue indexes. The angle ϕ that the major axisof the ellipse makes with respect to the x axis of our coordinate systemis the arctangent of the slope of the eigenvector. The slope of theeigenvector is just Δy/Δx.

As discussed above, the interpolation step requires determining a finetouch point, e.g., using data acquired from a flattened heat map, but isnot limited to the illustrative paraboloid model discussed above. Thepurpose of determining a fine touch point is to permit thepost-processor to provide better granularity in touch points, andspecifically, to provide granularity that exceeds the sensor'sintersections. Stated another way, the modeled and interpolated finetouch point can land directly on a row/column intersection, or anywherein between the intersections. There may be a tradeoff between theaccuracy of the model and its computational requirements; similarly,there may be a tradeoff between the accuracy of the model and itsability to provide an interpolated fine touch point that correspondswith the actual touch. Thus, in an embodiment, a model is selected torequire the smallest computational load while providing sufficientcorrespondence between the interpolated touch point and the actualtouch. In an embodiment, a model is selected to require sufficientcorrespondence between the interpolated touch point and the actualtouch, and the processing hardware is selected to accommodate thecomputational load of the model. In an embodiment, a model is selectedthat does not exceed the computational capacity of pre-selected hardwareand/or other software operating the touch interface.

Turning to the frame matching procedure, to properly track objectsmoving on the touch surface over time, it is important to match thecalculated touch points to each other across frame boundaries, and thus,e.g., to track objects moving on the touch surface as they move. Statedanother way, each calculated touch point in one frame should beidentified in, or have another disposition (e.g., removed) in, thesubsequent frame. While this is a fundamentally difficult problem, whichmay be insoluble in the general case, an embodiment can be implementedusing both geometry and the laws of physics. Because the items that arein contact with the touch surface are of finite size and move accordingto certain physical principles, certain cases can be ignored as beingoutside of plausible ranges. Moreover, in an embodiment, the frame rateshould be sufficient to permit object tracking (that is, frame-to-frametouch point tracking) with reasonable certainty. Thus, for example,where objects to be tracked are either known to move at a maximum rateacross the touch surface or the tracking is designed to track theobjects only up to a maximum rate, a frame rate can be selected thatwill permit tracking with reasonable certainty. For example, if amaximum rate of movement across the rows or columns of the touch surfaceis, e.g., 1000 rows or columns per second, then a frame rate of 1000 Hzwill “see” an object move no more than 1 row or column per frame. In anembodiment, touch point interpolation (as discussed above) can provide amore precise measure of the touch point location, and thus, intra-rowand intra-column positions are readily identifiable as described morefully herein.

Fingers and styluses have a minimum size and are unlikely to approacheach other closely enough to cause an ambiguous case. They also travelat speeds characteristic of the motion of a human arm and its parts(e.g., wrist, elbow, fingers, etc.), which bounds the problem. Becausethe touch surface of the presently disclosed sensor has a relativelyhigh update rate, which, in an embodiment, may be on the order of onekilohertz or more, fingers and styluses touching the surface cannot movevery far or at extreme angles during the update period from one frame tothe next. Because of the limited distances and angles, tracking can besomewhat simplified according to the present disclosure.

In an embodiment, tracking of objects moving on the touch surface overtime is performed by comparing data from one frame to one or more pastframes. In an embodiment, data concerning past frames (e.g., a heat map)may be maintained in a temporary buffer. In an embodiment, processeddata concerning past frames (e.g., field flattened heat map or fittedpolynomial coefficients) may be maintained in a temporary buffer. In anembodiment, the data concerning a past frame that is maintained in atemporary buffer may include, or may consist of, an interpolated finetouch point coordinate for each fine touch point in the prior frame,and, to the extent such exists, vectors concerning prior motion of thosefine touch points. The temporary buffer may retain data concerning oneor more past frames, and may cease to retain the data when it is nolonger relevant to later calculations.

In an embodiment, the frame matching process initially presumes that anobject's touch point in the current frame i is probably the touch pointin the prior frame (i.e., i−1) which is geometrically closest to it.

In an embodiment, data concerning the motion of a touch point (e.g.,velocity and direction) are determined and stored in connection with oneor more frames. In an embodiment, data concerning the motion of a touchpoint is used to predict a likely location for that touch point in thenext frame. Data concerning the motion of a touch point may comprise,for example, velocity or change in position, and may come from one ormore prior frames. In an embodiment, predicting a likely location in aframe is done by considering the motion between two frames—yielding aper-frame displacement and its direction. In an embodiment, predicting alikely location in a frame is done by considering the motion in three ormore frames. Using fine touch point positional information from three ormore frames may yield a more precise prediction as it can take intoaccount acceleration and changes of direction in addition to per-framedisplacement and direction. In an embodiment, more weight is assigned tomore recent frame data than to older frame data. A frame matchingprocess then may initially presume that an object's touch point in thecurrent frame i probably corresponds with the touch point in the priorframe (i.e., i−1) that is associated with the predicted likely locationclosest to the touch point in the current frame.

In an embodiment, data concerning the size (magnitude) of a touch point(e.g., the A and C coefficients of a paraboloid) is determined andstored in connection with one or more frames. A frame matching processmay initially presume that the size of a given object in the currentframe i probably corresponds with the size of that object in the priorframe (i.e., i−1).

In an embodiment, data concerning the change in size (magnitude) of atouch point over time are determined and stored in connection with oneor more frames. In an embodiment, data concerning the change in size ofa touch point in a frame (e.g., since the last frame, or over aplurality of frames) is used to predict a likely size for that touchpoint in the next frame. A frame matching process may initially presumethat an object in the current frame i probably corresponds with anobject in the prior frame (i.e., i−1) that is associated with thepredicted likely size nearest the size of the touch point in the currentframe.

In an embodiment, data concerning the change in rotational orientation(e.g., the B coefficient of a paraboloid) of a touch point over time aredetermined and stored in connection with one or more frames. In anembodiment, data concerning the rotational orientation of a touch pointin a frame (e.g., since the last frame, or over a plurality of frames)is used to predict a rotational orientation for that touch point in thenext frame. A frame matching process may initially presume that anobject in the current frame i probably corresponds with an object in theprior frame (i.e., i−1) that is associated with the predicted likelyrotational orientation nearest the rotational orientation of the touchpoint in the current frame. In an embodiment, the rotational orientationof a touch point could permit single touch point control (e.g., singlefinger control) of rotation, thus, for example, the rotation of onefinger on a screen could provide sufficient information to, for example,rotate a view—a function that traditionally requires two rotating pointsof contact with a touch surface. Using data describing rotationalorientation over time, rotational velocity can be computed. Similarly,data concerning rotational orientation or rotational velocity can beused to compute rotational acceleration. Thus, rotational velocity androtational acceleration both utilize rotational orientation. Rotationalorientation, rotational velocity and/or rotational acceleration may becomputed for a touch point and output by or used by the frame matchingprocess.

In an embodiment, heuristics for frame matching include changes indistance and in the velocity vectors of the touch points. In anembodiment, heuristics for frame matching include one or more of thefollowing:

-   -   a. an object's touch point in frame i+1 is probably the touch        point in frame i which is geometrically closest to it;    -   b. an object's touch point in frame i+1 is probably the touch        point in frame i which is closest to the point where it would be        predicted to be given the object's velocity history; and    -   c. an object's touch point in frame i+1 will be of a similar        size to its touch point in frame i.

Other combinations of historical data may be used without departing fromthe scope of this disclosure. In an embodiment, both prior positions andthe velocity histories may be used in a heuristic frame matchingprocess. In an embodiment, prior positions, the velocity histories andsize histories may be used in a heuristic frame matching process. In anembodiment, prior positions and other historical information may be usedin a heuristic frame matching process. In an embodiment, historicalinformation over a plurality of frames is used in a heuristic framematching process. Other combinations will be apparent to one of skill inthe art in view of the foregoing disclosure.

Fast Multi-Touch Noise Reduction

In an embodiment, methods and systems are provided to overcome certainconditions in which noise produces interference with, or phantom touchesin, the Fast Multi-Touch (FMT) sensor. In embodiments of the sensordescribed above, rows have a signal transmitted thereon and thetransmitted signal is coupled to columns in proximity to a touch ortouches, when a touch or touches are applied to or near the surface ofthe sensor. (In some cases, the touch or touches can cause a reductionof the row signal in the column.) The locations of touches aredetermined by reading the signals from the columns and determining therows in which they were produced.

When the sensor as described above is used in the presence of certainconditions (e.g., electromagnetic noise), it is possible for a column toreceive a signal from another source that is can be confused with aknown signal generated by one of the rows of the device. In such case,the device may report a phantom touch, determining that the signalreceived in the column is coming from a row, which in fact it is not.The present embodiments provide methods and devices for reducing oreliminating the occurrence of such phantom touches.

Thus, in an embodiment of the sensor, both the rows and the columns ofthe device are configured to both transmit unique signals, and also toreceive signals from the columns or rows of the device respectively. Inan embodiment, a detected signal from Row N in a given column may beconsidered a touch if that column's transmitted signal weresimultaneously detected in Row N. In other words, both the row and thecolumn must receive the other's transmitted signal in order for thedevice to report a touch at the intersection of the row and column. Asignal that is received in either the row or the column that is notmatched in this manner may be rejected as, for example, noise from anexternal source. In an alternative embodiment, both a detected signalfrom Row N in a given column, and a detected signal from the givencolumn in Row N may each be considered a touch regardless of whethermatching is found. While this configuration may not provide the benefitsof the matching described above, it may provide for a sensor withincreased sensitivity.

In an embodiment, unique signals may be transmitted on all rows andcolumns. In an embodiment, unique signals may be transmitted on each rowin one or more subsets of rows. In an embodiment, unique signals may betransmitted on each column in one or more subsets of columns. In anembodiment, all rows and columns are configured to detect the uniquesignals. In an embodiment, each row in one or more subsets of rows isconfigured to detect the unique signals. In an embodiment, each columnin one or more subsets of columns is configured to detect the uniquesignals.

FIG. 7 illustrates certain principles of a fast multi-touch sensor 700in accordance with an embodiment of the touch sensor. A transmitter andreceiver 702 are attached to each row and a transmitter and receiver 703are attached to each column. The transmitters shown at 702 may bediscrete from or part of the same element as the transmitters shown at703. Likewise, the receivers shown at 702 may be discrete from or partof the same element as the receivers shown at 703. The transmitters at702 and 703 may themselves be discrete elements or may simply comprise aconnection to a source of signal such as a signal generator, or may bepart of the signal generator. Likewise, the receivers shown at 702 and703 may be discrete elements or may simply comprise a connection to thesignal processor, or part of the signal processor. Reference no. 704represents both the transmitted row signals and the received row signalsand reference no. 705 represents both the transmitted column signals andthe received column signals. At least one subset of the transmitted rowsignals are designed to be orthogonal, i.e. separable anddistinguishable from each other. Likewise, at least one subset of thetransmitted column signals are designed to be orthogonal with respect toeach other. The receivers are designed to receive any of the transmittedsignals, or an arbitrary combination of them, while the signal processoris configured to individually measure the quantity of at least some ofthe orthogonal signals present on a column or row. In an embodiment,each of the orthogonal signals transmitted on the rows can be receivedand measured by the receiver/signal processor for a column, and each ofthe orthogonal signals transmitted on the columns can be received andmeasured by the receiver/signal processor for a row. As discussed above,the distinction between receiver and signal processor that is shown inthe drawing as a convenience for the reader, as is the distinctionbetween signal generator and transmitter. For example, a row or columnmay be connected directly to a signal processor, and thus the signalprocessor also acts as a receiver; similarly a row or column may beconnected to a signal generator, and thus, the signal generator wouldact as the transmitter. In an embodiment, all of the signal generatorsand receivers\signal processors could be integrated within the samemixed- signal ASIC.

Generally, in the present sensor, the signal coupled between the rowsand columns changes when they are not subject to a touch event versuswhen they are. In an embodiment, the rows and columns are configuredsuch that, when they are not subject to a touch event, a lower ornegligible amount of signal is coupled between them, whereas, when theyare subject to a touch event, a higher or non-negligible amount ofsignal is coupled between them. In an embodiment, the rows and columnsare configured such that, when they are subject to a touch event, alower or negligible amount of signal is coupled between them, whereas,when they are not subject to a touch event, a higher or non-negligibleamount of signal is coupled between them. In an embodiment, the signalcoupled between the rows and columns changes when they are not subjectto a touch event versus when they are. As discussed above, the wordtouch, or phrase touch event does not require a physical touching, butrather, requires an event affecting the sensor (e.g., but not noise) andwhich affects the level of coupled signal. In this respect, hovering isconsidered a touch event. Further, a “level” or “amount” of signal asused herein includes not only a discrete predetermined level but arelative amount of signal, a range of amounts of signal, an amount ofsignal that is determined dynamically at intervals of time or when atouch event determination is made, or any combination thereof. Thus, inan embodiment, the present sensor and configuration is able to identifytouch events resulting from a change in the signal coupled between oneor more rows and one or more columns.

As used below, for convenience of description, the terms transmittingconductor and receiving conductor will be used. The transmittingconductor may be a row or column carrying a signal e.g., from a signalgenerator. In this respect, “conductor” as used herein includes not onlyelectrical conductors but other paths on which signals flow. A receivingconductor may be a row or column carrying a signal resulting from thecoupling of a touch event when a touch event occurs in the proximity ofthe receiving conductor, and not carrying the signal resulting from thecoupling of a touch event when no touch event occurs in the proximity ofthe receiving conductor. In an embodiment, a receiver/signal processormeasures the quantity of each of the orthogonal transmitted signal on areceiving conductor which signals resulted from the coupling of a touchevent. Measuring the quantity allows for identification of a touchevent. The receiver/signal processor may comprise a DSP, a filter bank,or a combination thereof. In an embodiment, the receiver/signalprocessor is a comb filter providing bands corresponding to theorthogonal signals.

Because any touch event in proximity to a row-column intersection maychange both the row-signal present on the column, and the column-signalpresent on the row, in an embodiment, any signal on a column or row thatdoes not have a corresponding row or column counterpart may be rejected.In an embodiment, a row-signal received at a column receiver/signalprocessor is used in locating or identifying a touch event if acorresponding column-signal is received at a corresponding rowreceiver/signal processor. For example, a detected signal from Row R inColumn C is only considered to be caused by a touch event if Column C'stransmitted signal is also detected in Row R. In an embodiment, Column Cand Row R simultaneously transmit signals that are orthogonal to theother row and column signals, and orthogonal to each other. In anembodiment, Column C and Row R do not simultaneously transmit signals,but rather, each transmits its signal in an allotted time slice. In suchan embodiment, signals only require orthogonality from other signalstransmitted in the same time slice.

As illustrated, in an embodiment, a single signal generator may be usedto generate the orthogonal signals for both the rows and the columns,and a single signal processor may be used to process the receivedsignals from both the rows and the columns. In an embodiment, one signalgenerator is dedicated to generating row signals and a separate signalgenerator is dedicated to generating column signals. In an embodiment, aplurality of signal generators is dedicated to generating row signalsand the same, or a separate plurality of signal generators is dedicatedto generating column signals. Likewise, in an embodiment, one signalprocessor is dedicated to processing row signals and a separate signalprocessor is dedicated to processing column signals. In an embodiment, aplurality of signal processors are dedicated to processing row signalsand the same, or a separate plurality of signal processors are dedicatedto processing column signals.

In an embodiment, each receiving conductor is associated with a filterbank which acts as its receiver and signal processor, the filter bankbeing adapted to distinguish between a plurality of orthogonal signals.In an embodiment, a filter bank associated with a receiving-conductorrow is adapted to distinguish between all orthogonal signals that canresult from a touch event associated with that receiving-conductor row;likewise, a filter bank associated with a receiving conductor column isadapted to distinguish between all orthogonal signals that can resultfrom a touch event associated with that receiving-conductor column.

In an embodiment, each row and each column may be associated with asignal, and the signal associated with each row or column is unique andorthogonal with respect to the signal for every other row or column. Insuch an embodiment, it may be possible to “transmit” all row and columnsignals simultaneously. Where design or other constraints require, orwhere it is desirable to use fewer than one signal per row and column,time division multiplexing may be employed.

FIG. 8 illustrates a simplified example of a transmission scheme havingthree rows and four columns. In this illustrated embodiment, each rowand each column may be associated with a signal, and the signalassociated with each row or column is unique and orthogonal with respectto the signal for every other row or column. Specifically, signals A, Band C are associated with rows 1, 2 and 3, while signals D, E, F and Gare associated with columns 1, 2, 3 and 4. In this embodiment, it may bepossible to “transmit” all row and column signals simultaneously, eachrow and column acting as a transmitting conductor, and to simultaneouslyhave each row and column act as a receiving conductor, and thus beingable to process all the signals that can result from a touch eventsimultaneously.

FIG. 9 illustrates a simplified example of another transmission schemehaving three rows and four columns. In this illustrated embodiment, eachrow is associated with a signal, and the signal associated with each rowis unique and orthogonal with respect to the signal for every other row,and each column is associated with a signal, and the signal associatedwith each column is unique and orthogonal with respect to the signal forevery other column. In the illustrated embodiment, however, the signalsassociated with the rows are not all orthogonal with the signalsassociated with the columns, e.g., signal A is used for both rows andcolumns. Here, signals are transmitted on the rows, and received on thecolumns during a first time slice T1, and are transmitted on thecolumns, and received on the rows during a second time slice T2. In thismanner, only four, rather than seven orthogonal signals are required forthe implementation.

FIG. 10 illustrates a simplified example of yet another transmissionscheme having three rows and four columns. In this illustratedembodiment, each row and column is associated with a signal, and thesignal associated with each row and column is unique and orthogonal withrespect to the signal for every other row and column. In the illustratedembodiment, however, even though the signals associated with the rowsare all orthogonal with the signals associated with the columns, aconstraint or other design consideration may make it desirable to timedivision multiplex the transmission of the signals. Here again, signalsare transmitted on the rows, and received on the columns during a firsttime slice T1, and are transmitted on the columns, and received on therows during a second time slice T2. Such an embodiment may be useful,for example, where the range of frequency available for transmission maybe limited, and separation is important to reception. Accordingly, anassignment could be made as follows, permitting better separation forsimultaneously transmitted signals:

-   -   Row A: 5.001 MHz    -   Row B: 5.003 MHz    -   Row C: 5.005 MHz    -   Column D: 5.000 MHz    -   Column E: 5.002 MHz    -   Column F: 5.004 MHz    -   Column G: 5.006 MHz

FIG. 11 illustrates a simplified example of a transmission scheme havingthree rows and eight columns. In this illustrated embodiment, each rowis associated with a signal, and the signal associated with each row isunique and orthogonal with respect to the signal for every other row,but the columns share unique orthogonal signals that overlap with therow signals as illustrated. In the illustrated embodiment, three timeslices are employed to ensure that only unique orthogonal signals aresimultaneously transmitted, and therefore, a filter bank or other signalprocessor can locate a touch event in accordance with these teachings.

FIG. 12A shows an example of time division multiplexing applied withinsets of columns and also within sets of rows in a sensor having fourrows and eight columns. In this example, orthogonal frequencies A and Bare transmitted on a first set of rows and orthogonal frequencies C andD are transmitted on a first set of columns during time slice T1.Orthogonal frequencies A and B are transmitted on a second set of rowsand orthogonal frequencies C and D are transmitted on a second set ofcolumns during a subsequent time slice T2. Orthogonal frequencies C andD are transmitted on a third set of columns during a subsequent timeslice T3, and orthogonal frequencies C and D are transmitted on a fourthset of columns during a subsequent time slice T4. Optionally, orthogonalfrequencies A and B may be transmitted on the first or second set ofrows during time slices T3 and/or T4, for example, to provide greaterresolution of touch events in time.

FIG. 12B illustrates a simplified example of another transmission schemehaving four rows and eight columns. In this illustrated embodiment, onlytwo orthogonal signals, A and B are used. In the illustrated embodiment,six time slices are employed to ensure that while the two uniqueorthogonal signals can be simultaneously transmitted, neither cannot betransmitted on more than one transmitting conductor at once. Asillustrated, A and B are transmitted on rows 1 and 2 during the firsttime slice, columns 1 and 2 during the second time slice, columns 3 and4 during the third, and so forth.

Factors affecting the choice of orthogonal signal generation andtransmission scheme include, e.g., without limitation, the number ofrows and number of columns in the sensor, the desired resolution of thesensor, the material and dimensions of the rows and columns, availablesignal processing power, and the minimum acceptable latency of thesystem. Numerous other variations can be made, and are within the scopeand spirit of this disclosure and the attached claims. For example, itwill be apparent to a person of skill in the art the various tradeoffsthat can be made in selecting between the number of unique orthogonalsignals and the number of time slices employed by a given touchdetection system, provided however, that multiple signals aretransmitted in the same time slice, and each of those multiple signalsis orthogonal from all of the other signals transmitted in that timeslice.

As noted above, a column receiver Rx on a particular column may receivean orthogonal signal that was transmitted on one or more of the rowconductors, and the signal will be used by the signal processor todetermine the row conductor responsible for the touch event coupling,thus yielding a row-column coordinate. In addition to the orthogonalsignal transmitted on one or more rows, the column receiver Rx may “see”the signal originating from the column transmitter Tx, and its amplitudemay be quite great, and thus, may interfere with the processing of loweramplitude signals that have traversed portions of a row and column. Inan embodiment, the presently disclosed system and method provides forthe removal of the column transmitter Tx signal from the signalprocessed by the column receiver Rx. Thus, in an embodiment, theorthogonal signal sent by the column transmitter Tx may be subtractedfrom the signal received at the column receiver Rx. Such subtraction maybe provided electrically by a circuit comprising an inverter configuredsuch that the inverse of the signal transmitted by the columntransmitter Tx is added to the signal received by the column receiverRx, thereby subtracting the transmitted column signal from the receivedcolumn signal. Such a subtraction function may alternatively be providedin the signal processor (FIG. 7).

Dynamic Assignment of Possible Channels

The perceived quality of a touch sensor in a computer system depends ona high signal-to-noise ratio where-in user input signals are properlydiscerned from ambient electromagnetic noise. Such electromagnetic noisecan stem from other components within the computer system of which thetouch sensor is a part (e.g., an LCD information display) or fromartificial or natural signals in the user's external environment (e.g.,unwanted signal from a device's external AC power charger). Theseunwanted electromagnetic signals can be falsely detected by the touchsensor as user input and thereby produce false or noisy user commands.

In an embodiment, a system and method enables a touch sensor to reduceor eliminate such false or noisy readings and maintain a highsignal-to-noise ratio, even if it is proximate to interferingelectromagnetic noise from other computer system components or unwantedexternal signals. This method can also be used to dynamicallyreconfigure the signal modulation scheme governing select portions orthe entire surface-area of a touch sensor at a given point in time inorder to lower the sensor's total power consumption, while stilloptimizing the sensor's overall performance in terms of parallelism,latency, sample-rate, dynamic range, sensing granularity, etc.

Embodiments of the present system and method are particularlyadvantageous when applied to a capacitive touch sensor whose performancedepends on the accurate reading of electromagnetic signals, andparticularly for a capacitive touch sensor that employsfrequency-division multiplexing (FDM) to increase the scan-rate andlower the latency of reported touch input events to a computer system.In this respect, the present embodiments may be applied to sensors suchas those disclosed in Applicant's U.S. patent application Ser. No.13/841,436 filed on Mar. 15, 2013 entitled “Low-Latency Touch SensitiveDevice” and U.S. patent application Ser. No. 14/069,609 filed on Nov. 1,2013 entitled “Fast Multi-Touch Post Processing,” which contemplate acapacitive frequency-division multiplexing touch sensor as anembodiment.

Embodiments of the Dynamic Assignment Process Step 1: ReasonablyIdentify the Touch Signals and the Noise

The touch sensor can analyze all the signals it receives when it isknown that no user is touching the sensor, or if actual touch signalsare reasonably known (i.e. if it is known that some parts of the touchsurface are being touched while other parts are not being touched).

Such determinations of whether or not and where a touch sensor is beingtouched can be formed and strengthened through analyzing a combinationof readings from the sensor itself, other common computer input sensorslike accelerometers, the computer system's power status (e.g. if thecomputer is placed into “sleep mode,” etc.), the event stream fromcurrently running software applications on the computer system, etc.This analytic process of relying on data from more than one sensor in acomputer system to draw a conclusion about system state, the state ofsystem components, or the state of the user is commonly called “sensorfusion” in the art.

With an analytic judgment about known touches in-hand, all of the touchsensor's received signals can then be compared against signals receivedfor these known touches. The resulting differences between signals thesensor has measured, and what should have been measured (given what isknown about current or prior touch events) can then be used to mitigatethe noise and interference.

In an embodiment of this method, some of this measurement of interferingsignals can happen at design time, at least for the portions ofinterference thereof that will be predictable at design time. In anotherembodiment of this method, some of the measurement can happen atmanufacturing or testing time. In another embodiment, some of themeasurement can happen during a pre-use period when it is reasonablyknown that the user is not touching the touch sensor. In anotherembodiment, some of the measurement can occur when the user is touchingthe sensor at known positions. In another embodiment, some of themeasurement can occur at times between user touches when it is predictedby other sensors or algorithmically that the user is not touching thetouch surface.

In another embodiment, some of the measurement can occur statisticallyby software that can gauge the statistical patterns and likelihood of auser's touches. For example, the user-interface (UI) could have buttonsplaced at only certain positions on the touch surface, so that these arethe only places that a user is likely to be touching at a given time.When touched at one of these known positions, the difference between thetouch/no-touch states could be very obvious even in the presence ofnoise. In an embodiment, the UI can be designed such that a button mustbe held down for a certain defined period of time (perhaps indicated bythe display), yielding a pre-determined period over which the touch maybe detected even in the presence of noise. In another embodiment, aslider or two-dimensional “pointer” could be used instead of a button asthese UI controls require a user to follow an arbitrary path that iseither known by the UI ahead of time, or which can be dynamicallydetermined (to some extent) by other sensors on the device throughsensor fusion. In an embodiment, such a UI slider could be the single“slide-to-open” slider control commonly found on the “lock-screen” oftouch-friendly operating systems like but not limited to iOS, Android,other Linux variants, or Windows. In related embodiments, any suchunlocking gesture control can be used. In an embodiment, a virtualkeyboard provides known touch locations as the letters in a word can beeasily and accurately predicted through looking at the neighboringletters.

In an embodiment, such analysis could be performed on a touch sensor'sdiscrete touch controller. In another embodiment, such analysis could beperformed on other computer system components such as but not limited toASIC, MCU, FPGA, CPU, GPU, or SoC.

Step 2: Avoid The Interference

Once noisy readings are identified as “interference” based on knowntouch signals and/or via statistical inference as recounted in Step 1,such knowledge of electromagnetic interference can be used to avoidcollisions between certain portions of the frequency-, time- orcode-space where such noise can or will possibly be sensed by the touchsensor. Collisions between known touch signals and identifiedelectromagnetic interference can be avoided through a variety oftechniques or combinations of techniques such as but not limited to:

If there are identified signal frequencies that have no or littleinterference, then the touch sensor should be configured to use them. Ifthere are time slots that have little or no interference, then the touchsensor should be configured to use them. If there are codes that havelittle or no interference, then the touch sensor should be configured touse them. If there are combinations of frequency, time and code thathave little or no interference, then the touch sensor should beconfigured to use them.

For touch sensors that employ frequency division multiplexing (FDM), thesignal frequencies that the touch sensor employs do not have to becontiguous. If some parts of the frequency band are occupied byinterference, then the touch sensor can be configured to avoid thosefrequencies. If some parts of the frequency band are occupied byinterference at certain known times, then the touch sensor can beconfigured to avoid using those signal frequencies at those known times.If some parts of the frequency band are occupied by relatively staticinterference at certain known times, the signals transmitted by thetouch sensor can be modulated at those times in a fashion wherebydemodulation will cancel out or eliminate the known interference. Forexample, in an embodiment of this modulation technique, if theinterference is a steady sinusoid at some frequency of interest, binaryphase shift keying (BPSK) should be used to modulate the frequencyemitted by the touch sensor so that, when the opposite BPSK is used todemodulate the resulting sum of the signal received from the touchsensor and the interfering signal, an equal portion of the interferencehas been multiplied by the positive phase and an equal portion has beenmultiplied by the negative phase so that, when the signals areintegrated over the total reception period, the interference signal hassummed to something negligible. Other forms of modulation with similareffect are possible.

If a touch sensor using FDM employs a fast Fourier transform to performfrequency analysis, or a similar fast algorithm in which the number offrequency bins is constrained by the algorithm or nature of thealgorithm, the sensor can use a larger transform with a larger number ofbins (perhaps the next size up) so that there are extra possible receivefrequencies. The touch sensor can be configured prior to manufacturingwith the ability to transmit at any of these frequencies. In thismanner, if some of the frequency bins contain interference, these can beavoided in favor of frequencies that have little or no interference.

Step 3: Avoid Unwanted Hot-Spots

If some of the electromagnetic interference cannot be completelyeliminated through use of the aforementioned techniques, a touch sensorcan be configured to ensure that such noise is spread evenly across thesensor's surface-area, to minimize any operational problems posed by theremaining interference.

In an embodiment, a touch sensor can be configured and paired withcustom application programming interfaces (APIs) to ensure that morenoise-tolerant UI elements with respect to ensuring a good userexperience are placed on portions of the touch surface with more noise,and that portions of the UI that require near noise-free input commandsdue to the need for precision control are associated with parts of thetouch sensor's surface that are affected by little or no interference.In other embodiments, essentially the reverse of this concept isutilized. That is, a developer API can be used to flag UI elements whichthen dictate the placement of high performance modulation schemes on thetouch surface.

In another embodiment, unwanted electromagnetic noise can be mitigatedby remapping the timing, frequencies and codes assigned to touch sensorsignals. The division of these signals associated with the rows andcolumns of a touch sensor need not have a fixed relationship, and can bedynamically remapped as desired. For example, in an embodiment, a touchsensor that employs FDM may always transmit a sinusoid of a particularfrequency for a given row, or it may remap the frequencies that ittransmits dynamically. For example, if the touch sensor's transmitterand receiver are capable of operating at “n” different frequencies, andif “m” of those frequencies have been determined to contain asufficiently small amount of interference, and the number of touchsensor rows (simultaneously transmitted frequencies) is “r” (where “n”is greater than or equal to “m” which is greater than or equal to “r”),then the touch sensor can choose “r” frequencies out of the set of “m”and map those to the rows in a manner designed to minimize degradationto the user-experience. In another embodiment, the sensor's chosen setof operating frequencies can be re-mapped dynamically, every frame, in arandom or pseudo-random fashion so that there is a negligiblecorrelation of noise statistics between different portions of the touchsurface, over a noticeable time. More specifically, a touch sensor canchoose the “r” frequencies out of the “m”-possible if they have theleast noise or, it may choose among them dynamically and randomly (orpseudo-randomly) in a manner designed to minimize the correlation ofnoise statistics between different portions of the touch surface, over anoticeable time. Similar methods can be used for time slots, codes orother modulation schemes or combinations thereof.

In another embodiment, for a touch sensor that primarily employs FDM,where “m” frequencies, which have been determined to contain asufficiently small amount of interference, is greater than or equal tothe number of “r” frequencies required to simultaneously transmit aunique frequency on each sensor row, a touch sensor can employ a dynamicFDM modulation scheme that optimizes the latency and sample-rateperformance of specific portions of the touch sensor's surface-areabased on the known layout and requirements of UI controls. Here-in, theknown locations at a given point in time of UI controls demandinghigh-precision, low-latency user input are mapped onto correspondingportions of the surface-area of the touch sensor for which the signalmodulation scheme has been optimized at a given point in time for highperformance. Such dynamic mapping between the locations and performancerequirements of the computer system's software-defined UI controls andthe locations and performance requirements of the surface-area of thetouch sensor could be explicitly defined by the application developerbefore run-time or defined by operating system logic and analysis atrun-time of UI controls--with communication between the application,operating system, and touch-surface defined by application programminginterfaces (APIs). Simultaneously alongside these high performanceregions, other adjacent regions of the same surface-area could employlower performance frequency, time or code modulation schemes. Runningonly select regions of the surface-area of a touch sensor with amodulation scheme optimized for high performance in terms ofparallelism, latency, sample-rate, dynamic range, sensing granularity,etc. has the added benefit of potentially lowering the total energyconsumed by the touch sensor in order to both sense and process userinput, as only specific regions of the sensor are operated at demandingperformance levels—enabling the remainder of the surface-area to operatewith a modulation scheme that optimizes energy savings over performance.Such a dynamic modulation scheme can be updated and reoptimized as fastas every new frame of sensor input.

In another embodiment, for a touch sensor that primarily employs FDM,where the set of “m”-possible frequencies identified with the leastnoise is a number lower than the number of “r” unique sensor signalsrequired to assign a unique frequency to each row of the touch sensor,the sensor can be configured to employ a hybrid modulation approach thatcombines time, code or other modulation schemes with frequency division.In an embodiment of this method, the specific hybrid modulation approachcan be dynamically chosen and re-evaluated by the touch sensor—as fastas every new frame of sensor input—to optimize for the lowest latencyand the highest touch-event sample-rate across the entire sensor'ssurface-area. In another embodiment of this method, the specific hybridmodulation approach can be dynamically chosen and re-evaluated by thetouch sensor to optimize the latency and sample-rate performance ofspecific portions of the surface-area of the touch sensor based on theknown layout and requirements of UI controls. Here-in, the knownlocations at a given point in time of UI controls demandinghigh-precision, low-latency user input are mapped onto correspondingportions of the surface-area of the touch sensor for which the signalmodulation scheme has been optimized at a given point in time for highperformance in terms of parallelism, latency, sample-rate, dynamicrange, sensing granularity, etc. Such dynamic mapping between thelocations and performance requirements of the computer system'ssoftware-defined UI controls and the locations and performancerequirements of the surface-area of the touch sensor could be explicitlydefined by the application developer before run-time or defined byoperating system logic and analysis at run-time of UI controls—withcommunication between the application, operating system, andtouch-surface defined by application programming interfaces (APIs).Simultaneously alongside these high performance regions, other adjacentregions of the same surface-area could employ lower performancefrequency, time or code modulation schemes. Running only select regionsof the surface-area of a touch sensor with a modulation scheme optimizedfor high performance in terms of parallelism, latency, sample-rate,dynamic range, sensing granularity, etc. has the added benefit ofpotentially lowering the total energy consumed by the touch sensor inorder to both sense and process user input, as only specific regions ofthe sensor are operated at demanding performance levels—enabling theremainder of the surface-area to operate with a modulation scheme thatoptimizes energy savings over performance. Such a dynamic modulationscheme can be updated and reoptimized as fast as every new frame ofsensor input.

In another embodiment, for a touch sensor that primarily employs FDM,where the set of “m”-possible frequencies identified with the leastnoise is a number lower than the number of “r” unique sensor signalsrequired to assign a unique frequency to each row of the touch sensor,the sensor can be configured to enter a time-division multiplexing (TDM)mode for a given time period, choosing one of the frequencies in “m” andsampling rows and columns sequentially as is typical in a TDM approach.Switching a primarily FDM sensor to a pure TDM mode for a given timeperiod ensures accurate input, at the expense of the frame-rate andlatency of sensor readings.

In another embodiment, for a touch sensor that primarily employs FDM,where the set of “m”-possible frequencies identified with the leastnoise is a number lower than the number of “r” unique sensor signalsrequired to assign a unique frequency to each row of the touch sensor,the sensor can be configured to enter a hybrid FDM and TDM mode for agiven time period, choosing a select number of the frequencies in “m”and thereby sequentially sampling multiple rows and columns in parallelto improve the frame-rate and latency of sensor readings over theperformance limits of a purely sequential TDM mode. Such a hybrid FDMand TDM modulation scheme improves sensor parallelism and performance,while simultaneously mitigating the adverse impact of noisy readingsthat would have otherwise arisen from utilizing sensor signals outsideof “m” that real-time, historical, and/or statistical analysis of thesurrounding electromagnetic noise deemed more interference prone.

Step 4: Use Duplication of Sensing to Increase the Sensor'sSignal-to-Noise Ratio

A touch sensor can also utilize a number of techniques to decrease theinfluence of interference and other noise in the touch sensor. Forexample, in an embodiment for a touch sensor that employs FDM, a touchsensor could use multiple frequencies per row so that, even if thesensor cannot predict which frequency bins will be subject tointerference, then it can measure each row (or column) in multiple waysand gauge the least noisy measurement (or combination of measurements),and then use those.

In cases where it is difficult to decide whether a measurement has beenaffected by interference or not, a touch sensor could employ a votingscheme whereby a voting plurality of measurements, or a similarstatistical method, is used to determine which measurements to throwaway, which to keep and the best way to statistically and mathematicallycombine the ones it keeps to maximize the signal-to-noise+interferenceratio and thereby enhance the user experience. For example, in anembodiment, an FDM touch sensor subject to interference could transmitthree different frequencies on each row, (where the frequencies aresufficiently separated so that interference between them isstatistically unlikely) and measure the results. Then using atwo-out-of-three voting system, the sensor can determine which of thefrequencies has been degraded the most by interference and, eitherremove its measurement from consideration in the final measurement, orcombine the remaining two in a statistically plausible manner (givenwhat the sensor “knows” a priori about the interference and noisestatistics) or include all three and combine them in a statisticallyplausible manner, weighting the influence of each frequency measurementby the statistical likelihood of its degradation by noise andinterference.

Some methods that a touch sensor can employ in this manner include butare not limited to:

-   -   1. Using multiple frequencies per row. These frequencies could        be employed simultaneously or in sequence.    -   2. Transmitting from rows to columns, and from columns to rows        (either in sequence or simultaneously, as discussed in more        detail above.) This could also be combined with the use of        multiple frequencies above or with another combination of        modulation schemes.    -   3. Using CDMA on top of FDM, or some combination of modulation        schemes.

Here it should be noted that CDMA signals, unlike those commonlyemployed by FDM techniques, are fundamentally “unnatural” and thereforeare often more immune than FDM modulation schemes to a variety ofnaturally-occurring signals in a computer system's external environment.

User-Identification Techniques

In an embodiment, the fast multi-touch sensor is provided with theability to identify touches as coming from the same hand, differenthands of the same user, the same user, or different users. In anembodiment, the fast multi-touch sensor is provided with the ability toidentify touches as coming from a portion of an object linked to touchareas, either through capacitive touch points on a single object to helpdetermine its position and orientation or through a stylus held by auser who is also touching another area of the display simultaneouslywith a part of his/her body.

In the basic embodiment of the sensor initially discussed above, eachrow has a signal transmitter. The signal is coupled into nearby columnswhen a touch or touches are applied to the surface. The locations ofthese touches are determined by reading the signals from the columns andknowing in which rows they were produced.

When a user makes contact with the sensor or with a device within whichthe sensor is integrated, or comes within a certain distance of thesensor or otherwise causes a touch event, at more than one location,there will ordinarily be a certain amount of coupling that will occuracross touches made by the same user, as signals are transmitted by theuser's body from one touch location to the other. With reference to FIG.13, when a single touch or near touch is applied by a user's digit 1402at the intersection of row r1 with column c1, coupling will occurbetween row r1 and column c1. If a second, contemporaneous touch or neartouch is made by the user's second digit 1403 at the intersection of rowr2 and column c2, coupling will occur between row r2 and column c2.Additionally, weaker coupling may occur between row r1 and column c2, aswell as between r2 and column c1. In some embodiments, weaker couplingmay occur between the columns and between the rows.

These weaker, body-transmitted signals, which might otherwise bedismissed as “noise” or “cross talk,” can instead be used by the signalprocessor (FIG. 7) as an additional ‘signal’ to identify that a singleuser is responsible for both touches. In particular, to extend the aboveexample, the coupling between row r1 and column c2, as well as betweenrow r2 and column c1, might normally be considered ‘noise’, and filteredout (or otherwise ignored), to ensure a touch is not erroneouslyreported at the intersections of row r1 and column c2 or row r2 andcolumn c1. The weaker, body-transmitted coupling might still be filteredto ensure only accurate touch locations are reported, but alsointerpreted to allow the system to identify that the touches come fromthe same user. The sensor 400 may be configured to detect weaker,body-transmitted coupling transmitted from any digit of the user's hand,including but not limited to locations 1404, 1405, or 1406 in additionto 1403. The signal processor (FIG. 7) may be configured to use suchdetection to identify touches as coming from the same hand, differenthands of the same user, the same user, or different users.

In other embodiments of the of the touch sensor with useridentification, a signal generator can be coupled to the user elsewhere,such as in a handheld unit, a pad under their chair, or indeed on anedge of the device into which the sensor is integrated. Such a generatorcan be used to identify the user making a particular touch, in a mannersimilar to that described above. In other embodiments, the signalgenerator might be integrated into a stylus, pen, or other object.

The following are examples of the types of weaker coupling that can bedetected and used to identify touches as coming from the same hand, thesame user, or different users: coupling between a row or column beingtouched by a first one of the user's digits and a row or column beingtouched by a second one of the user's digits; coupling between a row orcolumn being touched by a user's digit and a row or column being touchedby another part of the user's body (such as his palm); coupling betweena row or column being touched by a part of the user's body (such as hisdigit or his palm) and a signal generator operatively connected to theuser's body; and coupling between a row or column being touched by apart of the user's body (such as his digit or his palm) and a signalgenerator integrated into a stylus or pen; and coupling between a row orcolumn being touched by a part of the user's body through a conductiveintermediary object, such as stylus or other tangible, and couplingbetween a row or column being touched by a part of the user's bodypossibly through a conductive intermediary object such as a stylus orother tangible. As used herein, “touch” includes events where there isphysical contact between a user and the disclosed sensor and also eventswhere there is no physical contact but an action by the user that occursin proximity to the sensor and is detected by the sensor.

The weaker couplings described above can be used to identify touches ascoming from the same hand, different hands of the same user, the sameuser, or different users. For example, the presence of a weaker couplingthat is relatively strong can be used to identify two touch events ascoming from the same hand, such as from two digits of the same hand(e.g., the index finger and the thumb) or a digit and a palm of the samehand. As another example, the presence of a weaker coupling that isrelatively weak (relative the previous example) can be used to identifytwo touch events as coming from different hands of the same person or ahand and another body part of the same person. As a third example, theabsence of a weaker coupling can be used to identify two touch events ascoming from different persons. Furthermore, the presence of a signalfrom a signal generator operatively connected to a user's body can beused to identify a touch as coming from a particular user, and theabsence of such signal can be used to identify a touch as not comingfrom a particular user.

Fast Multi-Touch Stylus

In certain embodiments of the fast multi-touch sensor, the sensor isconfigured to detect the position of a stylus and, optionally, its tiltangle and angle of rotation about its longitudinal axis as well. Suchembodiments begin with sensor hardware essentially as initiallydescribed above, and further utilize a stylus having a signaltransmitter near its tip, from which signals are transmitted which arecompatible (same or similar modulation scheme, similar frequency, etc.)with but orthogonal to the orthogonal signals that may be transmitted onthe rows or columns. A switch, which could be any kind of a switch,including, e.g., proximity detector or pressure sensor, in the tip ofthe stylus can be used to control when the transmitter is on or off. Thestylus can be configured such that, under normal operating conditions,the switch turns on the transmitter when the stylus is in contact withor within proximity to the fast multi-touch sensor's surface. In analternate embodiment, the stylus is configured such that it constantlytransmits a signal, and the state of the switch can change one or moreproperties of the signal, such as its frequency, amplitude, or the like.This allows the stylus to not only be used when it is in contact withthe surface of the touch sensitive device, but also when it is slightlyabove as well, providing a “hover” capability.

In an embodiment, the signal transmitted by the stylus is similar to theorthogonal signals which may be transmitted onto the rows as discussedabove, and the stylus can be treated essentially as an extra row.Signals emitted by the stylus are coupled into nearby columns and theamount of signal received on the columns can be used to determine theposition of the pen with respect to them.

To provide the ability to measure the position of the stylus in twodimensions, receivers can be placed on the rows of the FMT sensor, aswell as on the columns. The receivers on the rows do not need to be ascomplicated as those on the columns: the column receivers should beconfigured to pick up and discriminate between any of the signals thatare transmitted onto the rows. However, the row receivers only need tobe capable of picking up and discriminating between any signals that aretransmitted by the stylus or, in some embodiments, multiple styli.

In an embodiment, the signals transmitted by the stylus are be distinctfrom those transmitted onto the rows so that there is no confusionbetween them. If the row signals are modulated, the stylus signalsshould be similarly modulated to be compatible with the other receivers.In an embodiment, such modulation requires a time reference which themulti-touch sensor can be configured to provide to the stylus via acommunication channel. Such channel can be a radio link, an opticallink, an acoustic or ultrasonic link, or the like. In an embodiment, thestylus receives the row signals and synchronizes its modulation to them,with no other communication channel involved.

As the stylus transmits its signals, they are received by the column androw receivers. The signal strengths on the rows and columns are used todetermine the position of the stylus in two dimensions with respect tothe rows and columns. Stronger signal strengths are an indication thatthe stylus is in relatively close proximity to the sensor and weakersignal strengths are an indication that the stylus is farther away.Interpolation can be used to determine the position of the stylus to amuch finer resolution than the physical granularity of the rows andcolumns.

Stylus Tilt and Rotation

A more complex embodiment allows us to simultaneously measure both thetilt and rotation of the stylus as the user holds it, along withmeasuring the position of the stylus.

Instead of emitting a single signal, the stylus in this embodiment canemit multiple signals, each of which is transmitted from near the tip ofthe stylus, but from points spread around its circumference. While twosuch signals, 180 degrees apart, would provide some of the informationrequired, at least three signals (ideally 120 degrees apart) are neededto unambiguously measure the tilt and rotation of the stylus, and foursignals (ideally 90 degrees apart) would make the math and signalprocessing less cumbersome. The four-signal case is used in the examplesbelow.

Measuring Stylus Tilt

FIGS. 14 and 15 show two embodiments of a fast multi-touch stylus 1501having transmitters 1502 at its tip 1505. In the embodiment of FIG. 14,the transmitters 1502 are external on the tip 1505 while in theembodiment of FIG. 15 the transmitters 1502 are internal to the tip1505. The four transmitters 1502 are arranged around the circumferenceof the stylus 1501 and are oriented toward the North, East, South andWest, respectively along the planar surface of the fast multi-touchsensor 400. Imagine that the starting position of the pen is parallel tothe z-axis and perpendicular to the x- and the y-axis of the sensor'splanar surface. As the stylus is tilted toward the east as shown,rotating along the x- or y-axis to an angle a with respect to the planeof the sensor 400, the east-facing transmitter 1503 moves closer to thesurface of the sensor 400 in three-dimensional space relative to thenorth and south transmitters, and the west-facing transmitter movesfarther away from the sensor relative to the north and southtransmitters. This causes the orthogonal signal emitted by the easttransmitter to couple more strongly with the nearby rows and columns,which can be measured by their receivers within the fast multi-touchsensor. The orthogonal signal emitted by the west transmitter couplesless strongly with the nearby rows and columns, causing its signal toappear with lower strength in the receivers of those nearby rows andcolumns. By comparing the relative strengths of the east and westsignals, we can determine the tilt angle a of the stylus. Tilt in thenorth-south direction can be determined by a similar process with thenorth and south orthogonal signals. In an embodiment, a switch orpressure sensor 1504 in the tip 1505 of the stylus 1501 is used tocontrol when the transmitter is on or off. The stylus can be configuredsuch that, under normal operating conditions, the switch 1504 turns onthe transmitter when the stylus is in contact with or within proximityto the surface of the fast multi-touch sensor 400.

Measuring Stylus Rotation

Stylus rotation can be detected in a similar manner. As the x- andy-position of each of the stylus' four transmitters 1502 is rotated inparallel to the z-axis, the four transmitters on the pen will belinearly closer to or farther from the various rows and columns of thetouch surface. These different linear distances between the x- andy-position of the stylus' transmitters relative to the FMT's variousrows and columns result in different signal strengths picked up by theFMT's receivers. Rotating the stylus in parallel with the z-axis wouldchange these linear distances, and thus the associated signal strengths.The x- and y-rotation angle of the stylus can be inferred from thesedifferences in signal strengths.

Active Optical Stylus

Embodiments of the invention include a fast, accurate, low-latencystylus and sensor system that can be used for hand-written input on acomputer display or touch sensor. In an embodiment, the stylus providesinput that is fluid and natural, mimicking the experience of a pen orpencil. In this respect, the update rate of the system can be raised toover a kilohertz and the latency, from stylus movement to measuredposition and other parameters, can be lowered to less than onemillisecond. Along with measuring the position of the stylus, its tiltangle and rotation can be measured. It is noted that the Active OpticalStylus described herein is compatible with computer displays and touchsensors of virtually all designs and is not limited to use with the fastmulti-touch sensors described above.

The disclosed technique includes an optical method that uses InducedTotal Internal Reflection (ITIR). The technique allows a plurality ofstyli to simultaneously be used for input purposes. The sensor systemcan be placed on top of a computer display (such as an LCD or OLEDmonitor), and the inferred sensor position and other parameters overtime used to draw lines, curves, text, etc. on the computer display.

In an embodiment of the active optical stylus, the stylus emits light ina plurality of distinct patterns into the sensor surface. The sensorsurface is a thin flat sheet (or some two-dimensional manifold) ofmaterial that is transparent or translucent at the wavelength of thelight emitted from the stylus.

FIG. 16 shows a top view of the sensor sheet and the system as a whole.The stylus (indicated by the letter S) shines light in a plurality ofdistinct patterns into the sensor sheet (indicated by the letter A).Through direction changing means, which may comprise particles suspendedin a transparent medium, the sheet causes light at the pattern positionsto become trapped inside the sensor sheet, where it propagates in allhorizontal directions by total internal reflection. Angular filters(indicated by the letter B) only permit light in a small angle, i.e. arestricted angle, around the perpendicular to the sensor sheet edge topass through the filter. Linear light sensors (indicated by the letterC) detect where along their length that light is impinging on them. Inan embodiment, to detect the X, Y position of a single, simple stylus,it is only necessary to find the locations on the linear sensors onwhich the maximum amount of light is impinging. Light along the arrowlabeled “V” provides the vertical position of the stylus. Light alongthe arrow labeled “H” provides the horizontal position. Light in otherdirections is filtered and ignored.

FIG. 17 shows a side view of the sensor sheet. Normally, light enteringa transparent material having an index of refraction higher than thesurrounding medium will pass out the other side and be refracted at ashallower angle. It may not be possible for light emitted from theoutside to be trapped inside, unless something like a scattering mediumis in direct contact with the translucent material of a non-negligiblearea (as could happen in a frustrated total internal reflectionsituation). However, the non-negligible contact area required makes fora poor stylus because of the drag experienced by the contacting materialand the difficulty in building a stylus that can tilt and still maintainthe contact. A preferred embodiment uses a direction-changing meansinside the transparent material.

Inside the sheet, some of the light emitted by the stylus interacts withthe direction-changing means, which causes some light to become trappedin the sensor sheet and propagate outward away from the distinct patternof light which the stylus emitted into the sheet at that position. Thepropagating light travels to the edge of the sheet where it reaches anangular filter. Light that is perpendicular to the filter (and the edgeof the sheet) is allowed to pass to the linear light sensor.

FIG. 18 shows a side view of the sensor sheet. The direction-changingmeans inside the transparent material allows light emitted from thestylus to end up as light trapped inside the sheet, undergoing totalinternal reflection and propagating in all directions in the sheet.Light entering the sheet (solid arrow) enters the direction-changingmeans (the cloud shape). Light exits the direction-changing means inmany directions, some of which are within the angle at which totalinternal reflection can occur (dashed arrows). Some are outside theangle at which total internal reflection can occur (dotted lines). Thislight cannot be trapped and leaves the sensor sheet. Thedirection-changing means could come from scattering but, in thepreferred embodiment, it is a fluorescent or phosphorescent materialwhich absorbs the light emitted by the stylus, and emits light at adifferent wavelength, which propagates outward in all directions.

The linear light sensor measures the amount of light impinging on italong its length, which allows us to infer the position of the stylus.The position along the linear light sensor which receives the maximumamount of light corresponds to the projection of the stylus positionalong that dimension.

The system can not only measure the position of the stylus on the sensorsheet, but it can also infer its tilt and rotation, if the stylus emitsmore than a single ray of light. If the stylus emits multiple rays oflight, or perhaps cones or other shapes, the projection of these alongthe sides of the antenna sheet can be measured by the system, and thatdata used to simultaneously infer the position, tilt and rotation of thestylus.

Light Direction-Changing Property of the Material

Normally, light entering a thin transparent medium, like the sensorsurface, will exit out the other side, and none of it will get trappedinside and propagate by total internal reflection. In order for theentering light to be trapped and propagate inside, some means isrequired to change its direction. In one embodiment, the sensor surfacescatters some of the incoming light in different directions. Some ofthose directions are within the angle at which total internal reflectioncan occur. Scattering is not a preferred method because the there is noway to prevent the scattering from further changing the direction of thelight, which will lower the amount of light received by the linear lightsensors and also cause light to travel by non-straight-line paths, evenafter the first change of direction has occurred. Non-straight-linepaths will cause light to appear to come from incorrect directions andwill cause the system to yield false position readings.

The preferred direction-changing means is a one-time wavelength changingmeans, such as a fluorescent or phosphorescent material. Light emittedby the stylus at wavelength W1 enters the sensor sheet, where itinteracts with the one-time wavelength changing means. Said meansabsorbs a portion of that light and emits light at wavelength W2 inmultiple directions. Wavelength W1 could be in the ultraviolet portionof the electromagnetic spectrum. Wavelength W2 could be in the visibleor infrared portion of the spectrum. A portion of the light atwavelength W2 now propagates along the sensor sheet via total internalreflection and nothing otherwise impedes it because the one-timewavelength changing means does not appreciably affect wavelength W2.

Angular Filters

Light propagating through the sensor surface reaches the edge from amultitude of angles. In order to infer the position of the stylus'slight patterns inside the sensor surface, we want to restrict the linearlight sensor's field of vision to a specific direction. In anembodiment, an angular filter provides this function. In the preferredembodiment, with a rectangular sensor sheet and linear light sensors ontwo of the sides, we want to restrict the field of view of the lightsensors to directions perpendicular to the edges of the sensor sheet.This could be accomplished with a tiny set of “venetian blinds”, similarto the way that privacy screen for computer monitors restrict the viewto a narrow angle directly in front of the monitor.

Light impinging on the angular filter from directions outside theintended field of view should preferably be absorbed by the filter, orreflected in a manner such that the rejected light will not enter or besensed by any of the linear light sensors in the system.

FIG. 19 shows an angular filter (indicated by the letter B) in front ofa linear light sensor Indicated by the letter C), seen from the top ofthe system. The angular filter only permits light to enter that isperpendicular to the filter (and the linear light sensor). The filtercould be implemented in a manner similar to venetian blinds, with aplurality of perpendicular blades that block light which enters at otherangles. In the case, light along arrow 1901 is allowed to enter and passthrough the filter. Light along arrow 1902 is not permitted to enter,and is (preferentially) absorbed by the filter, or perhaps justreflected away. The linear light sensor can measure the amount of lightimpinging on it at a plurality of points along its length. The point atwhich the maximum amount of light impinges is probably the projection ofthe position of the stylus along the direction of the linear lightsensor.

Linear Light Sensors

The linear light sensors measure the amount of light impinging on themat a plurality of positions along their length. They could beimplemented by position sensitive detectors, linear CCD arrays, linearCMOS imager arrays, an array of photomultiplier tubes, and array ofindividual photodiodes, phototransistors, photo cells, or any othermeans of detecting light.

The Stylus

With reference to FIG. 20, the stylus 2001 is a pen-shaped device thatcan emit light in a plurality of distinct patterns into the sensor sheet2002 when the user holds it like a pen or pencil and draws on thesurface of the sensor sheet 2002. The projections of the patterns alongthe edges of the sensor sheet can be used to infer the position, tiltand rotation of the stylus. If multiple styli are desired, they can emittheir light one at a time, in a form of time-division multiplexing. Thiswould require some form of synchronization between the styli, whichcould be implemented by a variety of simple communication channels,including but not limited to a radio link, ultrasound or an opticalsignal. The optical signal could be generated by the computer displaybelow the sensor sheet, allowing the pens to be synchronized usingalmost no additional hardware.

The stylus could be constructed using a light source, such as alight-emitting diode, that is illuminated when a contact switch orpressure sensor senses that the stylus is in contact with the sensorsheet. Optical elements, such as lenses, diffraction gratings, lightpipes, splitters, etc. could take the light from a plurality of lightsources and create a different plurality of distinct patterns of lightwhich could be projected into the sensor sheet. In an embodiment, thestylus could also be a non-contact light source such as a laser.

Single Spot Embodiment

In a basic embodiment of the technology, the stylus emits a single rayor cone of light, probably coaxially with respect to the stylus body. Asingle ray of light will cause simple, point-like projections of thispattern along the sides of the sensor sheet, allowing us to infer theposition of the stylus. FIG. 21 shows the geometric projection of a spotemitted by a simple stylus along the edges of the sensor sheet. Themaxima of the light detected by the linear light sensors along theirlengths gives us the geometric projection of the illuminated spot on thesensor sheet. From this we can infer the sensor position.

If the stylus emits a cone-shaped beam, it will intersect the sensorsheet in a circle (if the stylus is held perpendicular to the surface)or in an ellipse (if the stylus is tilted away from perpendicular). Theprojections of these intersections will have different shapes and widthsallowing us to infer the tilt angle, as well as the angle relative tothe sensor sheet edges that the stylus is being held. FIG. 22 shows thegeometric projection of a spot emitted by a simple stylus along theedges of the sensor sheet. The maxima of the light detected by thelinear light sensors along their lengths gives us the geometricprojection of the illuminated spot on the sensor sheet. From this we caninfer the sensor position.

As shown in FIG. 23, if the stylus emits a cone of light, instead of aray of light, where that cone intersects the sensor sheet will cause anellipse. The projection of the ellipse may be different in one directionthan it is in the other, allowing us to infer the tilt of the stylus.

Multiple Spot Embodiment

If the stylus projects multiple patterns onto the sensor sheet, theprojections of these along the sides of the sensor sheet can be used toinfer the position, tilt and rotation angle of the stylus. As shown inFIG. 24, if both projections are wider than we would expect for a stylusheld perpendicular to the sensor sheet, and yet are nearly equal insize, it is probable that the stylus is tilted at a 45-degree angle tothe direction of the edges of the sensor sheet. The width of theprojections can be used to infer the tilt angle from the vertical. Thewider the projections, the greater the tilt.

With reference to FIG. 24, if the stylus emits multiple patterns oflight around its circumference, the projections of these along the edgesof the sensor sheet can allow us to infer the sensor tilt and also therotation around its axis, along with the location at which the stylus istouching the sensor sheet. The number and arrangement of the patternsprojected by the stylus must be carefully chosen. For example, thepatterns should not be evenly spaced around the circumference of thestylus because that might cause multiple rotation angles of the stylusto have the same projected light patterns along the edges of the sensorsheet. Even if this were the case, although the absolute rotation of thestylus could not always be measured, small relative rotations could bemeasured, which could still provide useful information to the userinterface. The most straightforward way to infer the stylus position,tilt and rotation from the geometric projections of its emitted patternsmight be to measure the projections for a wide variety of styluspositions, tilts and rotations, and then to map onto and interpolatebetween these to get from the projections back onto the stylusparameters. The two stylus patterns shown at A and B are identical,except that the stylus has been moved farther to the lower right androtated clockwise by 45 degrees.

Solar Blind UV

Sunlight contains many wavelengths of light, and these might interferewith operation of the stylus system if that is used in sunlight. Itwould be advantageous for the stylus to emit at a wavelength which iseither nonexistent or very weak in the solar spectrum as experienced atthe earth's surface. One possibility is for the stylus to emit light inthe solar blind region of the ultraviolet, where the oxygen in theearth's atmosphere absorbs most or all of those wavelengths. LEDs thatemit in the solar blind portion of the UV spectrum are available on thecommercial market.

A similar argument could be made for wavelengths of light from othersources (natural or artificial) that might impinge on the stylus systemand impede its use.

Multiple Stylus Embodiment

If it is desired to use multiple styli simultaneously, a method must beused to disambiguate the signals from each. For example, time-divisionmultiplexing can be used, in which case each stylus takes a turnemitting patterns (e.g., as shown in FIG. 20) into the sensor sheet.

Multiple styli could also use different direction-changing means, sothat each could emit at a different wavelength and these differentwavelengths could be distinguished after the direction-changing means bythe linear light sensors.

In certain embodiments, all of the styli emit at the same time with thesame wavelength, and disambiguate their contributions to the geometricprojections along the sides of the sensor sheet in software or firmware,using knowledge of the possible and likely trajectories of the styli asthey are being used.

The present system and methods are described above with reference toblock diagrams and operational illustrations of methods and devicescomprising a sensor capable of receiving and responding to user input.It is understood that each block of the block diagrams or operationalillustrations, and combinations of blocks in the block diagrams oroperational illustrations, may be implemented by means of analog ordigital hardware and computer program instructions. These computerprogram instructions may be provided to a processor of a general purposecomputer, special purpose computer, ASIC, or other programmable dataprocessing apparatus, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, implements the functions/acts specified in the block diagramsor operational block or blocks. In some alternate implementations, thefunctions/acts noted in the blocks may occur out of the order noted inthe operational illustrations. For example, two blocks shown insuccession may in fact be executed substantially concurrently or theblocks may sometimes be executed in the reverse order, depending uponthe functionality/acts involved.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

1. A method for determining a location of a touch event on a touch sensitive device, the touch sensitive device comprising a plurality of first conductors and a plurality of second conductors, the method comprising: a. simultaneously transmitting a first plurality of unique frequency orthogonal signals on each of the plurality of first conductors, each of the first plurality of unique frequency orthogonal signals being frequency orthogonal to each other of the first plurality of unique frequency orthogonal signals; b. determining a measurement of each of the simultaneously transmitted first plurality of unique frequency orthogonal signals present on each of the plurality of second conductors; c. simultaneously transmitting a second plurality of unique frequency orthogonal signals on each of the second plurality of conductors, each of the second plurality of unique frequency orthogonal column signals being frequency orthogonal to each other of the second plurality of unique frequency orthogonal signals; d. determining a measurement of each of the second plurality of simultaneously transmitted unique frequency orthogonal signals present on each of the first plurality of conductors; e. determining a location of a touch event on the touch sensitive device using the determined measurements of each of simultaneously transmitted first plurality of unique frequency orthogonal signals and the detected amount of each of the simultaneously transmitted second plurality of unique frequency orthogonal signals.
 2. The method of claim 1, wherein each of the simultaneously transmitted unique frequency orthogonal row signals is contained within the simultaneously transmitted unique frequency orthogonal column signals.
 3. The method of claim 1, wherein each of the simultaneously transmitted unique frequency orthogonal column signals is contained within the simultaneously transmitted unique frequency orthogonal row signals.
 4. The method of claim 1, wherein none of the simultaneously transmitted unique frequency orthogonal row signals is contained within the simultaneously transmitted unique frequency orthogonal column signals.
 5. The method of claim 1, wherein the simultaneously transmitted unique frequency orthogonal row signals is transmitted at the same time as the simultaneously transmitted unique frequency orthogonal column signals.
 6. The method of claim 1, wherein the simultaneously transmitted unique frequency orthogonal row signals are not transmitted at the same time as the simultaneously transmitted unique frequency orthogonal column signals.
 7. The method of claim 1, wherein the step of using comprises identifying the location of a touch event on the touch sensitive device where changes to the frequency orthogonal row signal for a first row are identified on a first column and changes to the frequency orthogonal column signal from the first column are identified on a first row.
 8. A device comprising: a. a first plurality of conductors and second plurality of conductors being arranged such that: i. the first plurality of conductors and the second plurality of conductors arranged so that at least one of the first plurality of conductors and at least one of the second plurality of conductors are located proximate to each other; b. signal generator adapted to generate simultaneously transmitting a first plurality of unique frequency orthogonal signals on each of the plurality of first conductors, each of the first plurality of unique frequency orthogonal signals being frequency orthogonal to each other of the first plurality of unique frequency orthogonal signals; c. simultaneously generate a first plurality of unique frequency orthogonal signals, each unique frequency orthogonal signal being frequency orthogonal to each of the other unique frequency orthogonal row signals, the row signal generator being operatively connected to the plurality of row conductors such that at least a unique one of the plurality of unique frequency orthogonal row signals is generated on each of the plurality of row conductors; d. column signal decoder operatively connected to the plurality of column conductors, the column signal decoder adapted to detect a level for each of the plurality of unique frequency orthogonal row signals on each of the plurality of column conductors; e. column signal generator adapted to simultaneously generate a plurality of unique frequency orthogonal column signals, each unique frequency orthogonal column signal being frequency orthogonal to each of the other unique frequency orthogonal column signals, the column signal generator being operatively connected to the plurality of column conductors such that at least a unique one of the plurality of unique frequency orthogonal column signals is generated on each of the plurality of row conductors; f. row signal decoder operatively connected to the plurality of row conductors, the row signal decoder adapted to detect a level for each of the plurality of unique frequency orthogonal column signals on each of the plurality of row conductors; and g. touch event locator for determining coordinates of a touch event based, at least in part, on the detected levels for each of the plurality of unique frequency orthogonal row signals on each of the plurality of column conductors and for each of the plurality of unique frequency orthogonal column signals on each of the plurality of row conductors, wherein the touch event locator rejects coordinates as a touch event when a detected level of the unique frequency orthogonal row signal for a specific row on a specific column does not correspond to a detected level of the unique frequency orthogonal column signal for the specific column on the specific row.
 9. The device of claim 8, wherein each of the plurality of unique frequency orthogonal row signals is contained within the plurality of unique frequency orthogonal column signals.
 10. The method of claim 8, wherein each of the plurality of unique frequency orthogonal column signals is contained within the plurality of unique frequency orthogonal row signals.
 11. The device of claim 8, wherein none of the plurality of unique frequency orthogonal row signals is contained within the plurality of unique frequency orthogonal column signals.
 12. The device of claim 8, wherein the row signal generator generates the plurality of unique frequency orthogonal row signals at the same time that the column signal generator generates the plurality of unique frequency orthogonal column signals.
 13. The device of claim 8, wherein the row signal generator does not generate the plurality of unique frequency orthogonal row signals at the same time that the column signal generator is generating the plurality of unique frequency orthogonal column signals.
 14. A low-latency touch sensitive device comprising: first and second pluralities of conductors being arranged such that: each of the paths of the conductors of the first plurality of conductors cross each of the paths of the conductors of the second plurality of conductors, and wherein when the touch sensitive device is not being touched, a first level of signal is coupled between conductors of the plurality of row conductors and conductors of the plurality of column conductors, and when the touch sensitive device is being touched, a second level of signal is coupled between at least one of the conductors of the plurality of row conductors and at least one of the conductors of the plurality of column conductors; at least one signal generator adapted to generate a plurality of unique frequency orthogonal signals, each unique frequency orthogonal signal being frequency orthogonal to each of the other unique frequency orthogonal signals; first transmitter associated with the first plurality of conductors, the first transmitter being adapted to simultaneously transmit a first subset of the plurality of unique frequency orthogonal signals on each of the first plurality of conductors respectively such that at least a unique one of the signals from the first subset is generated on each of the first plurality of conductors; second transmitter associated with the second plurality of conductors, the second transmitter being adapted to transmit a second subset of the plurality of unique frequency orthogonal signals on each of the second plurality of conductors simultaneously such that at least a unique one of the signals from the second subset is generated on each of the second plurality of conductors; first receiver associated with the first plurality of conductors, the first receiver being adapted to receive signals present on each of the first plurality of conductors; second receiver associated with the second plurality of conductors, the second receiver being adapted to receive signals present on each of the second plurality of conductors; and signal processor adapted to decode the signals received by the first receiver and the signals received by the second receiver, and for each of the signals received by the first receiver and the signals received by the second receiver, to determine which of the unique frequency orthogonal signals meet the second level of signal, the signal processor further adapted to correlate signals received by the first receiver that meet the second level of signal to signals received by the second receiver that meet the second level of signal, wherein the signal processor is adapted to determine that a touch has occurred at or near an intersection of a conductor of the first plurality of conductors and a conductor of the second plurality of conductors when the first receiver receives a signal that is determined to represent a potential touch at or near the conductor of the first plurality of conductors that corresponds to a signal received by the second receiver that is determined to represent a potential touch at or near the conductor of the second plurality of conductors.
 15. The low-latency touch sensitive device according to claim 14, further comprising a circuit for electrically subtracting a signal sent by the first transmitter from a signal received at the first receiver.
 16. The low-latency touch sensitive device according to claim 14, wherein the signal processor is configured to subtract a signal sent by the first transmitter from a signal received at the first receiver.
 17. The low-latency touch sensitive device according to claim 14, wherein the first and second transmitters are discrete transmitters.
 18. The low-latency touch sensitive device according to claim 14, wherein the first and second transmitters are the same transmitter.
 19. The low-latency touch sensitive device according to claim 14, wherein the first and second transmitters form part of the at least one signal generator.
 20. The low-latency touch sensitive device according to claim 14, wherein the first and second transmitters are separate from the at least one signal generator. 